datastore

package
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 Go to latest Published: Aug 10, 2016 License: Apache-2.0 Imports: 24 Imported by: 0

Documentation

Overview

Package datastore provides a client for Google Cloud Datastore.

Basic Operations

Entities are the unit of storage and are associated with a key. A key consists of an optional parent key, a string application ID, a string kind (also known as an entity type), and either a StringID or an IntID. A StringID is also known as an entity name or key name.

It is valid to create a key with a zero StringID and a zero IntID; this is called an incomplete key, and does not refer to any saved entity. Putting an entity into the datastore under an incomplete key will cause a unique key to be generated for that entity, with a non-zero IntID.

An entity's contents are a mapping from case-sensitive field names to values. Valid value types are:

  • signed integers (int, int8, int16, int32 and int64),
  • bool,
  • string,
  • float32 and float64,
  • []byte (up to 1 megabyte in length),
  • any type whose underlying type is one of the above predeclared types,
  • *Key,
  • GeoPoint,
  • time.Time (stored with microsecond precision),
  • structs whose fields are all valid value types,
  • slices of any of the above.

Slices of structs are valid, as are structs that contain slices. However, if one struct contains another, then at most one of those can be repeated. This disqualifies recursively defined struct types: any struct T that (directly or indirectly) contains a []T.

The Get and Put functions load and save an entity's contents. An entity's contents are typically represented by a struct pointer.

Example code: 

type Entity struct {
	Value string
}

func main() {
	ctx := context.Background()

	// Create a datastore client. In a typical application, you would create
	// a single client which is reused for every datastore operation.
	dsClient, err := datastore.NewClient(ctx, "my-project")
	if err != nil {
		// Handle error.
	}

	k := datastore.NewKey(ctx, "Entity", "stringID", 0, nil)
	e := new(Entity)
	if err := dsClient.Get(ctx, k, e); err != nil {
		// Handle error.
	}

	old := e.Value
	e.Value = "Hello World!"

	if _, err := dsClient.Put(ctx, k, e); err != nil {
		// Handle error.
	}

	fmt.Printf("Updated value from %q to %q\n", old, e.Value)
}

GetMulti, PutMulti and DeleteMulti are batch versions of the Get, Put and Delete functions. They take a []*Key instead of a *Key, and may return a datastore.MultiError when encountering partial failure.

Properties

An entity's contents can be represented by a variety of types. These are typically struct pointers, but can also be any type that implements the PropertyLoadSaver interface. If using a struct pointer, you do not have to explicitly implement the PropertyLoadSaver interface; the datastore will automatically convert via reflection. If a struct pointer does implement that interface then those methods will be used in preference to the default behavior for struct pointers. Struct pointers are more strongly typed and are easier to use; PropertyLoadSavers are more flexible.

The actual types passed do not have to match between Get and Put calls or even across different calls to datastore. It is valid to put a *PropertyList and get that same entity as a *myStruct, or put a *myStruct0 and get a *myStruct1. Conceptually, any entity is saved as a sequence of properties, and is loaded into the destination value on a property-by-property basis. When loading into a struct pointer, an entity that cannot be completely represented (such as a missing field) will result in an ErrFieldMismatch error but it is up to the caller whether this error is fatal, recoverable or ignorable.

By default, for struct pointers, all properties are potentially indexed, and the property name is the same as the field name (and hence must start with an upper case letter). Fields may have a `datastore:"name,options"` tag. The tag name is the property name, which must be one or more valid Go identifiers joined by ".", but may start with a lower case letter. An empty tag name means to just use the field name. A "-" tag name means that the datastore will ignore that field. If options is "noindex" then the field will not be indexed. If the options is "" then the comma may be omitted. There are no other recognized options.

All fields are indexed by default. Strings or byte slices longer than 1500 bytes cannot be indexed; fields used to store long strings and byte slices must be tagged with "noindex" or they will cause Put operations to fail.

Example code: 

// A and B are renamed to a and b.
// A, C and J are not indexed.
// D's tag is equivalent to having no tag at all (E).
// I is ignored entirely by the datastore.
// J has tag information for both the datastore and json packages.
type TaggedStruct struct {
	A int `datastore:"a,noindex"`
	B int `datastore:"b"`
	C int `datastore:",noindex"`
	D int `datastore:""`
	E int
	I int `datastore:"-"`
	J int `datastore:",noindex" json:"j"`
}

Structured Properties

If the struct pointed to contains other structs, then the nested or embedded structs are flattened. For example, given these definitions: 

type Inner1 struct {
	W int32
	X string
}

type Inner2 struct {
	Y float64
}

type Inner3 struct {
	Z bool
}

type Outer struct {
	A int16
	I []Inner1
	J Inner2
	Inner3
}

then an Outer's properties would be equivalent to those of: 

type OuterEquivalent struct {
	A     int16
	IDotW []int32  `datastore:"I.W"`
	IDotX []string `datastore:"I.X"`
	JDotY float64  `datastore:"J.Y"`
	Z     bool
}

If Outer's embedded Inner3 field was tagged as `datastore:"Foo"` then the equivalent field would instead be: FooDotZ bool `datastore:"Foo.Z"`.

If an outer struct is tagged "noindex" then all of its implicit flattened fields are effectively "noindex".

The PropertyLoadSaver Interface

An entity's contents can also be represented by any type that implements the PropertyLoadSaver interface. This type may be a struct pointer, but it does not have to be. The datastore package will call Load when getting the entity's contents, and Save when putting the entity's contents. Possible uses include deriving non-stored fields, verifying fields, or indexing a field only if its value is positive.

Example code: 

type CustomPropsExample struct {
	I, J int
	// Sum is not stored, but should always be equal to I + J.
	Sum int `datastore:"-"`
}

func (x *CustomPropsExample) Load(ps []datastore.Property) error {
	// Load I and J as usual.
	if err := datastore.LoadStruct(x, ps); err != nil {
		return err
	}
	// Derive the Sum field.
	x.Sum = x.I + x.J
	return nil
}

func (x *CustomPropsExample) Save() ([]datastore.Property, error) {
	// Validate the Sum field.
	if x.Sum != x.I + x.J {
		return errors.New("CustomPropsExample has inconsistent sum")
	}
	// Save I and J as usual. The code below is equivalent to calling
	// "return datastore.SaveStruct(x)", but is done manually for
	// demonstration purposes.
	return []datastore.Property{
		{
			Name:  "I",
			Value: int64(x.I),
		},
		{
			Name:  "J",
			Value: int64(x.J),
		},
	}
}

The *PropertyList type implements PropertyLoadSaver, and can therefore hold an arbitrary entity's contents.

Queries

Queries retrieve entities based on their properties or key's ancestry. Running a query yields an iterator of results: either keys or (key, entity) pairs. Queries are re-usable and it is safe to call Query.Run from concurrent goroutines. Iterators are not safe for concurrent use.

Queries are immutable, and are either created by calling NewQuery, or derived from an existing query by calling a method like Filter or Order that returns a new query value. A query is typically constructed by calling NewQuery followed by a chain of zero or more such methods. These methods are:

  • Ancestor and Filter constrain the entities returned by running a query.
  • Order affects the order in which they are returned.
  • Project constrains the fields returned.
  • Distinct de-duplicates projected entities.
  • KeysOnly makes the iterator return only keys, not (key, entity) pairs.
  • Start, End, Offset and Limit define which sub-sequence of matching entities to return. Start and End take cursors, Offset and Limit take integers. Start and Offset affect the first result, End and Limit affect the last result. If both Start and Offset are set, then the offset is relative to Start. If both End and Limit are set, then the earliest constraint wins. Limit is relative to Start+Offset, not relative to End. As a special case, a negative limit means unlimited.

Example code: 

type Widget struct {
	Description string
	Price       int
}

func printWidgets(ctx context.Context, client *datastore.Client) {
	q := datastore.NewQuery("Widget").
		Filter("Price <", 1000).
		Order("-Price")
	for t := dsClient.Run(ctx, q); ; {
		var x Widget
		key, err := t.Next(&x)
		if err == datastore.Done {
			break
		}
		if err != nil {
			// Handle error.
		}
		fmt.Printf("Key=%v\nWidget=%#v\n\n", key, x)
	}
}

Transactions

Client.RunInTransaction runs a function in a transaction.

Example code: 

type Counter struct {
	Count int
}

func incCount(ctx context.Context, client *datastore.Client) {
	var count int
	key := datastore.NewKey(ctx, "Counter", "singleton", 0, nil)
	err := dsClient.RunInTransaction(ctx, func(tx *datastore.Transaction) error {
		var x Counter
		if err := tx.Get(key, &x); err != nil && err != datastore.ErrNoSuchEntity {
			return err
		}
		x.Count++
		if _, err := tx.Put(key, &x); err != nil {
			return err
		}
		count = x.Count
	}, nil)
	if err != nil {
		// Handle error.
	}
	// The value of count is only valid once the transaction is successful
	// (RunInTransaction has returned nil).
	fmt.Printf("Count=%d\n", count)
}
Example (Auth)

TODO(jbd): Document other authorization methods and refer to them here. 

package main

import (
	"log"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() *datastore.Client {
	ctx := context.Background()
	// Use Google Application Default Credentials to authorize and authenticate the client.
	// More information about Application Default Credentials and how to enable is at
	// https://developers.google.com/identity/protocols/application-default-credentials.
	client, err := datastore.NewClient(ctx, "project-id")
	if err != nil {
		log.Fatal(err)
	}
	// Use the client (see other examples).
	return client
}

Output:
Example (BasicEntity) 
package main

import (
	"time"
)

func main() {
	// [START basic_entity]
	type Task struct {
		Type            string
		Done            bool
		Priority        float64
		Description     string `datastore:",noindex"`
		PercentComplete float64
		Created         time.Time
	}
	task := &Task{
		Type:            "Personal",
		Done:            false,
		Priority:        4,
		Description:     "Learn Cloud Datastore",
		PercentComplete: 10.0,
		Created:         time.Now(),
	}
	// [END basic_entity]
	_ = task // Use the task in a datastore Put operation.
}

Output:
Example (ExplodingProperties) 
package main

import (
	"time"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	// [START exploding_properties]
	task := &Task{
		Tags:          []string{"fun", "programming", "learn"},
		Collaborators: []string{"alice", "bob", "charlie"},
		Created:       time.Now(),
	}
	// [END exploding_properties]
	_ = task // Use the task in a datastore Put operation.
}

Output:
Example (MetadataKinds) 
package main

import (
	"log"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	// [START kind_run_query]
	query := datastore.NewQuery("__kind__").KeysOnly()
	keys, err := client.GetAll(ctx, query, nil)
	if err != nil {
		log.Fatalf("client.GetAll: %v", err)
	}

	kinds := make([]string, 0, len(keys))
	for _, k := range keys {
		kinds = append(kinds, k.Name())
	}
	// [END kind_run_query]
}

Output:
Example (MetadataNamespaces) 
package main

import (
	"log"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	// [START namespace_run_query]
	const (
		startNamespace = "g"
		endNamespace   = "h"
	)
	query := datastore.NewQuery("__namespace__").
		Filter("__key__ >=", startNamespace).
		Filter("__key__ <", endNamespace).
		KeysOnly()
	keys, err := client.GetAll(ctx, query, nil)
	if err != nil {
		log.Fatalf("client.GetAll: %v", err)
	}

	namespaces := make([]string, 0, len(keys))
	for _, k := range keys {
		namespaces = append(namespaces, k.Name())
	}
	// [END namespace_run_query]
}

Output:
Example (MetadataProperties) 
package main

import (
	"log"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	// [START property_run_query]
	query := datastore.NewQuery("__property__").KeysOnly()
	keys, err := client.GetAll(ctx, query, nil)
	if err != nil {
		log.Fatalf("client.GetAll: %v", err)
	}

	props := make(map[string][]string) // Map from kind to slice of properties.
	for _, k := range keys {
		prop := k.Name()
		kind := k.Parent().Name()
		props[kind] = append(props[kind], prop)
	}
	// [END property_run_query]
}

Output:
Example (MetadataPropertiesForKind) 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	// [START property_by_kind_run_query]
	kindKey := datastore.NewKey(ctx, "__kind__", "Task", 0, nil)
	query := datastore.NewQuery("__property__").Ancestor(kindKey)

	type Prop struct {
		Repr []string `datastore:"property_representation"`
	}

	var props []Prop
	keys, err := client.GetAll(ctx, query, &props)
	// [END property_by_kind_run_query]
	_ = err  // Check error.
	_ = keys // Use keys to find property names, and props for their representations.
}

Output:
Example (Properties) 
package main

import (
	"time"
)

func main() {
	// [START properties]
	type Task struct {
		Type            string
		Done            bool
		Priority        int
		Description     string `datastore:",noindex"`
		PercentComplete float64
		Created         time.Time
	}
	task := &Task{
		Type:            "Personal",
		Done:            false,
		Priority:        4,
		Description:     "Learn Cloud Datastore",
		PercentComplete: 10.0,
		Created:         time.Now(),
	}
	// [END properties]
	_ = task // Use the task in a datastore Put operation.
}

Output:
Example (SliceProperties) 
package main

import ()

func main() {
	// [START array_value]
	type Task struct {
		Tags          []string
		Collaborators []string
	}
	task := &Task{
		Tags:          []string{"fun", "programming"},
		Collaborators: []string{"alice", "bob"},
	}
	// [END array_value]
	_ = task // Use the task in a datastore Put operation.
}

Output:

Index

Examples

Constants

View Source 
const ScopeDatastore = "https://www.googleapis.com/auth/datastore"

ScopeDatastore grants permissions to view and/or manage datastore entities

Variables

View Source 
var (
	// ErrInvalidEntityType is returned when functions like Get or Next are
	// passed a dst or src argument of invalid type.
	ErrInvalidEntityType = errors.New("datastore: invalid entity type")
	// ErrInvalidKey is returned when an invalid key is presented.
	ErrInvalidKey = errors.New("datastore: invalid key")
	// ErrNoSuchEntity is returned when no entity was found for a given key.
	ErrNoSuchEntity = errors.New("datastore: no such entity")
)
View Source 
var Done = errors.New("datastore: query has no more results")

Done is returned when a query iteration has completed.

View Source 
var ErrConcurrentTransaction = errors.New("datastore: concurrent transaction")

ErrConcurrentTransaction is returned when a transaction is rolled back due to a conflict with a concurrent transaction.

Functions

func LoadStruct 

func LoadStruct(dst interface{}, p []Property) error

LoadStruct loads the properties from p to dst. dst must be a struct pointer.

func WithNamespace 

func WithNamespace(parent context.Context, namespace string) context.Context

WithNamespace returns a new context that limits the scope its parent context with a Datastore namespace.

Types

type Client 

type Client struct {
	// contains filtered or unexported fields
}

Client is a client for reading and writing data in a datastore dataset.

func NewClient 

func NewClient(ctx context.Context, projectID string, opts ...option.ClientOption) (*Client, error)

NewClient creates a new Client for a given dataset. If the project ID is empty, it is derived from the DATASTORE_PROJECT_ID environment variable. If the DATASTORE_EMULATOR_HOST environment variable is set, client will use its value to connect to a locally-running datastore emulator.

func (*Client) AllocateIDs 

func (c *Client) AllocateIDs(ctx context.Context, keys []*Key) ([]*Key, error)

AllocateIDs accepts a slice of incomplete keys and returns a slice of complete keys that are guaranteed to be valid in the datastore

func (*Client) Close 

func (c *Client) Close()

Close closes the Client.

func (*Client) Count 

func (c *Client) Count(ctx context.Context, q *Query) (int, error)

Count returns the number of results for the given query.

The running time and number of API calls made by Count scale linearly with with the sum of the query's offset and limit. Unless the result count is expected to be small, it is best to specify a limit; otherwise Count will continue until it finishes counting or the provided context expires.

func (*Client) Delete 

func (c *Client) Delete(ctx context.Context, key *Key) error

Delete deletes the entity for the given key.

Example 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	key := datastore.NewKey(ctx, "Task", "sampletask", 0, nil)
	// [START delete]
	err := client.Delete(ctx, key)
	// [END delete]
	_ = err // Make sure you check err.
}

Output:

func (*Client) DeleteMulti 

func (c *Client) DeleteMulti(ctx context.Context, keys []*Key) error

DeleteMulti is a batch version of Delete.

Example 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	var taskKeys []*datastore.Key // Populated with incomplete keys.
	// [START batch_delete]
	err := client.DeleteMulti(ctx, taskKeys)
	// [END batch_delete]
	_ = err // Make sure you check err.
}

Output:

func (*Client) Get 

func (c *Client) Get(ctx context.Context, key *Key, dst interface{}) error

Get loads the entity stored for key into dst, which must be a struct pointer or implement PropertyLoadSaver. If there is no such entity for the key, Get returns ErrNoSuchEntity.

The values of dst's unmatched struct fields are not modified, and matching slice-typed fields are not reset before appending to them. In particular, it is recommended to pass a pointer to a zero valued struct on each Get call.

ErrFieldMismatch is returned when a field is to be loaded into a different type than the one it was stored from, or when a field is missing or unexported in the destination struct. ErrFieldMismatch is only returned if dst is a struct pointer.

Example 
package main

import (
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	taskKey := datastore.NewKey(ctx, "Task", "sampleTask", 0, nil)
	// [START lookup]
	var task Task
	err := client.Get(ctx, taskKey, &task)
	// [END lookup]
	_ = err // Make sure you check err.
}

Output:

func (*Client) GetAll 

func (c *Client) GetAll(ctx context.Context, q *Query, dst interface{}) ([]*Key, error)

GetAll runs the provided query in the given context and returns all keys that match that query, as well as appending the values to dst.

dst must have type *[]S or *[]*S or *[]P, for some struct type S or some non- interface, non-pointer type P such that P or *P implements PropertyLoadSaver.

As a special case, *PropertyList is an invalid type for dst, even though a PropertyList is a slice of structs. It is treated as invalid to avoid being mistakenly passed when *[]PropertyList was intended.

The keys returned by GetAll will be in a 1-1 correspondence with the entities added to dst.

If q is a “keys-only” query, GetAll ignores dst and only returns the keys.

The running time and number of API calls made by GetAll scale linearly with with the sum of the query's offset and limit. Unless the result count is expected to be small, it is best to specify a limit; otherwise GetAll will continue until it finishes collecting results or the provided context expires.

func (*Client) GetMulti 

func (c *Client) GetMulti(ctx context.Context, keys []*Key, dst interface{}) error

GetMulti is a batch version of Get.

dst must be a []S, []*S, []I or []P, for some struct type S, some interface type I, or some non-interface non-pointer type P such that P or *P implements PropertyLoadSaver. If an []I, each element must be a valid dst for Get: it must be a struct pointer or implement PropertyLoadSaver.

As a special case, PropertyList is an invalid type for dst, even though a PropertyList is a slice of structs. It is treated as invalid to avoid being mistakenly passed when []PropertyList was intended.

Example 
package main

import (
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	var taskKeys []*datastore.Key // Populated with incomplete keys.
	// [START batch_lookup]
	var tasks []*Task
	err := client.GetMulti(ctx, taskKeys, &tasks)
	// [END batch_lookup]
	_ = err // Make sure you check err.
}

Output:

func (*Client) NewTransaction 

func (c *Client) NewTransaction(ctx context.Context, opts ...TransactionOption) (*Transaction, error)

NewTransaction starts a new transaction.

func (*Client) Put 

func (c *Client) Put(ctx context.Context, key *Key, src interface{}) (*Key, error)

Put saves the entity src into the datastore with key k. src must be a struct pointer or implement PropertyLoadSaver; if a struct pointer then any unexported fields of that struct will be skipped. If k is an incomplete key, the returned key will be a unique key generated by the datastore.

Example 
package main

import (
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	// [START entity_with_parent]
	parentKey := datastore.NewKey(ctx, "TaskList", "default", 0, nil)
	key := datastore.NewIncompleteKey(ctx, "Task", parentKey)

	task := Task{
		Type:        "Personal",
		Done:        false,
		Priority:    4,
		Description: "Learn Cloud Datastore",
	}

	// A complete key is assigned to the entity when it is Put.
	var err error
	key, err = client.Put(ctx, key, &task)
	// [END entity_with_parent]
	_ = err // Make sure you check err.
}

Output:
Example (Upsert) 
package main

import (
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	task := &Task{} // Populated with appropriate data.
	key := datastore.NewIncompleteKey(ctx, "Task", nil)
	// [START upsert]
	key, err := client.Put(ctx, key, task)
	// [END upsert]
	_ = err // Make sure you check err.
	_ = key // key is the complete key for the newly stored task
}

Output:

func (*Client) PutMulti 

func (c *Client) PutMulti(ctx context.Context, keys []*Key, src interface{}) ([]*Key, error)

PutMulti is a batch version of Put.

src must satisfy the same conditions as the dst argument to GetMulti.

Example 
package main

import (
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	// [START batch_upsert]
	tasks := []*Task{
		{
			Type:        "Personal",
			Done:        false,
			Priority:    4,
			Description: "Learn Cloud Datastore",
		},
		{
			Type:        "Personal",
			Done:        false,
			Priority:    5,
			Description: "Integrate Cloud Datastore",
		},
	}
	keys := []*datastore.Key{
		datastore.NewIncompleteKey(ctx, "Task", nil),
		datastore.NewIncompleteKey(ctx, "Task", nil),
	}

	keys, err := client.PutMulti(ctx, keys, tasks)
	// [END batch_upsert]
	_ = err  // Make sure you check err.
	_ = keys // keys now has the complete keys for the newly stored tasks.
}

Output:

func (*Client) Run 

func (c *Client) Run(ctx context.Context, q *Query) *Iterator

Run runs the given query in the given context.

func (*Client) RunInTransaction 

func (c *Client) RunInTransaction(ctx context.Context, f func(tx *Transaction) error, opts ...TransactionOption) (*Commit, error)

RunInTransaction runs f in a transaction. f is invoked with a Transaction that f should use for all the transaction's datastore operations.

f must not call Commit or Rollback on the provided Transaction.

If f returns nil, RunInTransaction commits the transaction, returning the Commit and a nil error if it succeeds. If the commit fails due to a conflicting transaction, RunInTransaction retries f with a new Transaction. It gives up and returns ErrConcurrentTransaction after three failed attempts.

If f returns non-nil, then the transaction will be rolled back and RunInTransaction will return the same error. The function f is not retried.

Note that when f returns, the transaction is not committed. Calling code must not assume that any of f's changes have been committed until RunInTransaction returns nil.

Since f may be called multiple times, f should usually be idempotent. Note that Transaction.Get is not idempotent when unmarshaling slice fields.

type Commit 

type Commit struct{}

Commit represents the result of a committed transaction.

func (*Commit) Key 

func (c *Commit) Key(p *PendingKey) *Key

Key resolves a pending key handle into a final key.

type Cursor 

type Cursor struct {
	// contains filtered or unexported fields
}

Cursor is an iterator's position. It can be converted to and from an opaque string. A cursor can be used from different HTTP requests, but only with a query with the same kind, ancestor, filter and order constraints.

The zero Cursor can be used to indicate that there is no start and/or end constraint for a query.

func DecodeCursor 

func DecodeCursor(s string) (Cursor, error)

Decode decodes a cursor from its base-64 string representation.

func (Cursor) String 

func (c Cursor) String() string

String returns a base-64 string representation of a cursor.

type ErrFieldMismatch 

type ErrFieldMismatch struct {
	StructType reflect.Type
	FieldName  string
	Reason     string
}

ErrFieldMismatch is returned when a field is to be loaded into a different type than the one it was stored from, or when a field is missing or unexported in the destination struct. StructType is the type of the struct pointed to by the destination argument passed to Get or to Iterator.Next.

func (*ErrFieldMismatch) Error 

func (e *ErrFieldMismatch) Error() string

type GeoPoint 

type GeoPoint struct {
	Lat, Lng float64
}

GeoPoint represents a location as latitude/longitude in degrees.

func (GeoPoint) Valid 

func (g GeoPoint) Valid() bool

Valid returns whether a GeoPoint is within [-90, 90] latitude and [-180, 180] longitude.

type Iterator 

type Iterator struct {
	// contains filtered or unexported fields
}

Iterator is the result of running a query.

func (*Iterator) Cursor 

func (t *Iterator) Cursor() (Cursor, error)

Cursor returns a cursor for the iterator's current location.

Example 
package main

import (
	"log"
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	cursorStr := ""
	// [START cursor_paging]
	const pageSize = 5
	query := datastore.NewQuery("Tasks").Limit(pageSize)
	if cursorStr != "" {
		cursor, err := datastore.DecodeCursor(cursorStr)
		if err != nil {
			log.Fatalf("Bad cursor %q: %v", cursorStr, err)
		}
		query = query.Start(cursor)
	}

	// Read the tasks.
	var tasks []Task
	var task Task
	it := client.Run(ctx, query)
	_, err := it.Next(&task)
	for err == nil {
		tasks = append(tasks, task)
		_, err = it.Next(&task)
	}
	if err != datastore.Done {
		log.Fatalf("Failed fetching results: %v", err)
	}

	// Get the cursor for the next page of results.
	nextCursor, err := it.Cursor()
	// [END cursor_paging]
	_ = err        // Check the error.
	_ = nextCursor // Use nextCursor.String as the next page's token.
}

Output:

func (*Iterator) Next 

func (t *Iterator) Next(dst interface{}) (*Key, error)

Next returns the key of the next result. When there are no more results, Done is returned as the error.

If the query is not keys only and dst is non-nil, it also loads the entity stored for that key into the struct pointer or PropertyLoadSaver dst, with the same semantics and possible errors as for the Get function.

type Key 

type Key struct {
	// contains filtered or unexported fields
}

Key represents the datastore key for a stored entity, and is immutable.

func DecodeKey 

func DecodeKey(encoded string) (*Key, error)

DecodeKey decodes a key from the opaque representation returned by Encode.

func NewIncompleteKey 

func NewIncompleteKey(ctx context.Context, kind string, parent *Key) *Key

NewIncompleteKey creates a new incomplete key. kind cannot be empty.

Example 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	// [START incomplete_key]
	taskKey := datastore.NewIncompleteKey(ctx, "Task", nil)
	// [END incomplete_key]
	_ = taskKey // Use the task key for datastore operations.
}

Output:

func NewKey 

func NewKey(ctx context.Context, kind, name string, id int64, parent *Key) *Key

NewKey creates a new key. kind cannot be empty. At least one of name and id must be zero. If both are zero, the key returned is incomplete. parent must either be a complete key or nil.

Example 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	// [START named_key]
	taskKey := datastore.NewKey(ctx, "Task", "sampletask", 0, nil)
	// [END named_key]
	_ = taskKey // Use the task key for datastore operations.
}

Output:
Example (WithMultipleParents) 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	// [START key_with_multilevel_parent]
	userKey := datastore.NewKey(ctx, "User", "alice", 0, nil)
	parentKey := datastore.NewKey(ctx, "TaskList", "default", 0, userKey)
	taskKey := datastore.NewKey(ctx, "Task", "sampleTask", 0, parentKey)
	// [END key_with_multilevel_parent]
	_ = taskKey // Use the task key for datastore operations.
}

Output:
Example (WithParent) 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	// [START key_with_parent]
	parentKey := datastore.NewKey(ctx, "TaskList", "default", 0, nil)
	taskKey := datastore.NewKey(ctx, "Task", "sampleTask", 0, parentKey)
	// [END key_with_parent]
	_ = taskKey // Use the task key for datastore operations.
}

Output:

func (*Key) Encode 

func (k *Key) Encode() string

Encode returns an opaque representation of the key suitable for use in HTML and URLs. This is compatible with the Python and Java runtimes.

func (*Key) Equal 

func (k *Key) Equal(o *Key) bool

func (*Key) GobDecode 

func (k *Key) GobDecode(buf []byte) error

func (*Key) GobEncode 

func (k *Key) GobEncode() ([]byte, error)

func (*Key) ID 

func (k *Key) ID() int64

func (*Key) Incomplete 

func (k *Key) Incomplete() bool

Complete returns whether the key does not refer to a stored entity.

func (*Key) Kind 

func (k *Key) Kind() string

func (*Key) MarshalJSON 

func (k *Key) MarshalJSON() ([]byte, error)

func (*Key) Name 

func (k *Key) Name() string

func (*Key) Namespace 

func (k *Key) Namespace() string

func (*Key) Parent 

func (k *Key) Parent() *Key

func (*Key) SetParent 

func (k *Key) SetParent(v *Key)

func (*Key) String 

func (k *Key) String() string

String returns a string representation of the key.

func (*Key) UnmarshalJSON 

func (k *Key) UnmarshalJSON(buf []byte) error

type MultiError 

type MultiError []error

MultiError is returned by batch operations when there are errors with particular elements. Errors will be in a one-to-one correspondence with the input elements; successful elements will have a nil entry.

func (MultiError) Error 

func (m MultiError) Error() string

type PendingKey 

type PendingKey struct {
	// contains filtered or unexported fields
}

PendingKey represents the key for newly-inserted entity. It can be resolved into a Key by calling the Key method of Commit.

type Property 

type Property struct {
	// Name is the property name.
	Name string
	// Value is the property value. The valid types are:
	//	- int64
	//	- bool
	//	- string
	//	- float64
	//	- *Key
	//	- time.Time
	//	- GeoPoint
	//	- []byte (up to 1 megabyte in length)
	// Value can also be:
	//	- []interface{} where each element is one of the above types
	// This set is smaller than the set of valid struct field types that the
	// datastore can load and save. A Value's type must be explicitly on
	// the list above; it is not sufficient for the underlying type to be
	// on that list. For example, a Value of "type myInt64 int64" is
	// invalid. Smaller-width integers and floats are also invalid. Again,
	// this is more restrictive than the set of valid struct field types.
	//
	// A Value will have an opaque type when loading entities from an index,
	// such as via a projection query. Load entities into a struct instead
	// of a PropertyLoadSaver when using a projection query.
	//
	// A Value may also be the nil interface value; this is equivalent to
	// Python's None but not directly representable by a Go struct. Loading
	// a nil-valued property into a struct will set that field to the zero
	// value.
	Value interface{}
	// NoIndex is whether the datastore cannot index this property.
	// If NoIndex is set to false, []byte and string values are limited to
	// 1500 bytes.
	NoIndex bool
}

Property is a name/value pair plus some metadata. A datastore entity's contents are loaded and saved as a sequence of Properties. Each property name must be unique within an entity.

func SaveStruct 

func SaveStruct(src interface{}) ([]Property, error)

SaveStruct returns the properties from src as a slice of Properties. src must be a struct pointer.

type PropertyList 

type PropertyList []Property

PropertyList converts a []Property to implement PropertyLoadSaver.

func (*PropertyList) Load 

func (l *PropertyList) Load(p []Property) error

Load loads all of the provided properties into l. It does not first reset *l to an empty slice.

func (*PropertyList) Save 

func (l *PropertyList) Save() ([]Property, error)

Save saves all of l's properties as a slice of Properties.

type PropertyLoadSaver 

type PropertyLoadSaver interface {
	Load([]Property) error
	Save() ([]Property, error)
}

PropertyLoadSaver can be converted from and to a slice of Properties.

type Query 

type Query struct {
	// contains filtered or unexported fields
}

Query represents a datastore query.

Example 
package main

import (
	"log"
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	client, err := datastore.NewClient(ctx, "project-id")
	if err != nil {
		log.Fatal(err)
	}

	// Count the number of the post entities.
	q := datastore.NewQuery("Post")
	n, err := client.Count(ctx, q)
	if err != nil {
		log.Fatal(err)
	}
	log.Printf("There are %d posts.", n)

	// List the posts published since yesterday.
	yesterday := time.Now().Add(-24 * time.Hour)
	q = datastore.NewQuery("Post").Filter("PublishedAt >", yesterday)
	it := client.Run(ctx, q)
	// Use the iterator.
	_ = it

	// Order the posts by the number of comments they have recieved.
	datastore.NewQuery("Post").Order("-Comments")

	// Start listing from an offset and limit the results.
	datastore.NewQuery("Post").Offset(20).Limit(10)
}

Output:
Example (Basic) 
package main

import (
	"fmt"
	"log"
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	// [START basic_query]
	query := datastore.NewQuery("Task").
		Filter("Done =", false).
		Filter("Priority >=", 4).
		Order("-Priority")
	// [END basic_query]
	// [START run_query]
	it := client.Run(ctx, query)
	for {
		var task Task
		_, err := it.Next(&task)
		if err == datastore.Done {
			break
		}
		if err != nil {
			log.Fatalf("Error fetching next task: %v", err)
		}
		fmt.Printf("Task %q, Priority %d\n", task.Description, task.Priority)
	}
	// [END run_query]
}

Output:
Example (CompositeFilter) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START composite_filter]
	query := datastore.NewQuery("Task").Filter("Done =", false).Filter("Priority =", 4)
	// [END composite_filter]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (InequalitySort) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START inequality_sort]
	query := datastore.NewQuery("Task").
		Filter("Priority >", 3).
		Order("Priority").
		Order("Created")
	// [END inequality_sort]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (InvalidInequalitySortA) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START inequality_sort_invalid_not_same]
	query := datastore.NewQuery("Task").
		Filter("Priority >", 3).
		Order("Created")
	// [END inequality_sort_invalid_not_same]
	_ = query // The query is invalid.
}

Output:
Example (InvalidInequalitySortB) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START inequality_sort_invalid_not_first]
	query := datastore.NewQuery("Task").
		Filter("Priority >", 3).
		Order("Created").
		Order("Priority")
	// [END inequality_sort_invalid_not_first]
	_ = query // The query is invalid.
}

Output:
Example (KeyFilter) 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	// [START key_filter]
	key := datastore.NewKey(ctx, "Task", "someTask", 0, nil)
	query := datastore.NewQuery("Task").Filter("__key__ >", key)
	// [END key_filter]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (Kindless) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	var lastSeenKey *datastore.Key
	// [START kindless_query]
	query := datastore.NewQuery("").Filter("__key__ >", lastSeenKey)
	// [END kindless_query]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (PropertyFilter) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START property_filter]
	query := datastore.NewQuery("Task").Filter("Done =", false)
	// [END property_filter]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (SortAscending) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START ascending_sort]
	query := datastore.NewQuery("Task").Order("created")
	// [END ascending_sort]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (SortDescending) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START descending_sort]
	query := datastore.NewQuery("Task").Order("-created")
	// [END descending_sort]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (SortMulti) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START multi_sort]
	query := datastore.NewQuery("Task").Order("-priority").Order("created")
	// [END multi_sort]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (Unindexed) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START unindexed_property_query]
	query := datastore.NewQuery("Tasks").Filter("Description =", "A task description")
	// [END unindexed_property_query]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:

func NewQuery 

func NewQuery(kind string) *Query

NewQuery creates a new Query for a specific entity kind.

An empty kind means to return all entities, including entities created and managed by other App Engine features, and is called a kindless query. Kindless queries cannot include filters or sort orders on property values.

func (*Query) Ancestor 

func (q *Query) Ancestor(ancestor *Key) *Query

Ancestor returns a derivative query with an ancestor filter. The ancestor should not be nil.

Example 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	// [START ancestor_query]
	ancestor := datastore.NewKey(ctx, "TaskList", "default", 0, nil)
	query := datastore.NewQuery("Task").Ancestor(ancestor)
	// [END ancestor_query]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:

func (*Query) Distinct 

func (q *Query) Distinct() *Query

Distinct returns a derivative query that yields de-duplicated entities with respect to the set of projected fields. It is only used for projection queries.

Example 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START distinct_query]
	query := datastore.NewQuery("Task").
		Project("Priority", "PercentComplete").
		Distinct().
		Order("Type").Order("Priority")
	// [END distinct_query]
	_ = query // Use client.Run or client.GetAll to execute the query.

	// [START distinct_on_query]
	// DISTINCT ON not supported in Go API
	// [END distinct_on_query]
}

Output:

func (*Query) End 

func (q *Query) End(c Cursor) *Query

End returns a derivative query with the given end point.

func (*Query) EventualConsistency 

func (q *Query) EventualConsistency() *Query

EventualConsistency returns a derivative query that returns eventually consistent results. It only has an effect on ancestor queries.

Example 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	// [START eventual_consistent_query]
	ancestor := datastore.NewKey(ctx, "TaskList", "default", 0, nil)
	query := datastore.NewQuery("Task").Ancestor(ancestor).EventualConsistency()
	// [END eventual_consistent_query]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:

func (*Query) Filter 

func (q *Query) Filter(filterStr string, value interface{}) *Query

Filter returns a derivative query with a field-based filter. The filterStr argument must be a field name followed by optional space, followed by an operator, one of ">", "<", ">=", "<=", or "=". Fields are compared against the provided value using the operator. Multiple filters are AND'ed together. Field names which contain spaces, quote marks, or operator characters should be passed as quoted Go string literals as returned by strconv.Quote or the fmt package's %q verb.

Example (ArrayEquality) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START array_value_equality]
	query := datastore.NewQuery("Task").
		Filter("Tag =", "fun").
		Filter("Tag =", "programming")
	// [END array_value_equality]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (ArrayInequality) 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START array_value_inequality_range]
	query := datastore.NewQuery("Task").
		Filter("Tag >", "learn").
		Filter("Tag <", "math")
	// [END array_value_inequality_range]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (Inequality) 
package main

import (
	"time"

	"cloud.google.com/go/datastore"
)

func main() {
	// [START inequality_range]
	query := datastore.NewQuery("Task").
		Filter("Created >", time.Date(1990, 1, 1, 0, 0, 0, 0, time.UTC)).
		Filter("Created <", time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC))
	// [END inequality_range]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:
Example (InvalidInequality) 
package main

import (
	"time"

	"cloud.google.com/go/datastore"
)

func main() {
	// [START inequality_invalid]
	query := datastore.NewQuery("Task").
		Filter("Created >", time.Date(1990, 1, 1, 0, 0, 0, 0, time.UTC)).
		Filter("Priority >", 3)
	// [END inequality_invalid]
	_ = query // The query is invalid.
}

Output:
Example (Mixed) 
package main

import (
	"time"

	"cloud.google.com/go/datastore"
)

func main() {
	// [START equal_and_inequality_range]
	query := datastore.NewQuery("Task").
		Filter("Priority =", 4).
		Filter("Done =", false).
		Filter("Created >", time.Date(1990, 1, 1, 0, 0, 0, 0, time.UTC)).
		Filter("Created <", time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC))
	// [END equal_and_inequality_range]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:

func (*Query) KeysOnly 

func (q *Query) KeysOnly() *Query

KeysOnly returns a derivative query that yields only keys, not keys and entities. It cannot be used with projection queries.

Example 
package main

import (
	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	// [START keys_only_query]
	query := datastore.NewQuery("Task").KeysOnly()
	// [END keys_only_query]
	// [START run_keys_only_query]
	keys, err := client.GetAll(ctx, query, nil)
	// [END run_keys_only_query]
	_ = err  // Make sure you check err.
	_ = keys // Keys contains keys for all stored tasks.
}

Output:

func (*Query) Limit 

func (q *Query) Limit(limit int) *Query

Limit returns a derivative query that has a limit on the number of results returned. A negative value means unlimited.

Example 
package main

import (
	"cloud.google.com/go/datastore"
)

func main() {
	// [START limit]
	query := datastore.NewQuery("Task").Limit(5)
	// [END limit]
	_ = query // Use client.Run or client.GetAll to execute the query.
}

Output:

func (*Query) Offset 

func (q *Query) Offset(offset int) *Query

Offset returns a derivative query that has an offset of how many keys to skip over before returning results. A negative value is invalid.

func (*Query) Order 

func (q *Query) Order(fieldName string) *Query

Order returns a derivative query with a field-based sort order. Orders are applied in the order they are added. The default order is ascending; to sort in descending order prefix the fieldName with a minus sign (-). Field names which contain spaces, quote marks, or the minus sign should be passed as quoted Go string literals as returned by strconv.Quote or the fmt package's %q verb.

func (*Query) Project 

func (q *Query) Project(fieldNames ...string) *Query

Project returns a derivative query that yields only the given fields. It cannot be used with KeysOnly.

Example 
package main

import (
	"log"
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	// [START projection_query]
	query := datastore.NewQuery("Task").Project("Priority", "PercentComplete")
	// [END projection_query]
	// [START run_query_projection]
	var priorities []int
	var percents []float64
	it := client.Run(ctx, query)
	for {
		var task Task
		if _, err := it.Next(&task); err == datastore.Done {
			break
		} else if err != nil {
			log.Fatal(err)
		}
		priorities = append(priorities, task.Priority)
		percents = append(percents, task.PercentComplete)
	}
	// [END run_query_projection]
}

Output:

func (*Query) Start 

func (q *Query) Start(c Cursor) *Query

Start returns a derivative query with the given start point.

func (*Query) Transaction 

func (q *Query) Transaction(t *Transaction) *Query

Transaction returns a derivative query that is associated with the given transaction.

All reads performed as part of the transaction will come from a single consistent snapshot. Furthermore, if the transaction is set to a serializable isolation level, another transaction cannot concurrently modify the data that is read or modified by this transaction.

type Transaction 

type Transaction struct {
	// contains filtered or unexported fields
}

Transaction represents a set of datastore operations to be committed atomically.

Operations are enqueued by calling the Put and Delete methods on Transaction (or their Multi-equivalents). These operations are only committed when the Commit method is invoked. To ensure consistency, reads must be performed by using Transaction's Get method or by using the Transaction method when building a query.

A Transaction must be committed or rolled back exactly once.

Example 
package main

import (
	"log"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

func main() {
	ctx := context.Background()
	client, err := datastore.NewClient(ctx, "project-id")
	if err != nil {
		log.Fatal(err)
	}
	const retries = 3

	// Increment a counter.
	// See https://cloud.google.com/appengine/articles/sharding_counters for
	// a more scalable solution.
	type Counter struct {
		Count int
	}

	key := datastore.NewKey(ctx, "counter", "CounterA", 0, nil)

	for i := 0; i < retries; i++ {
		tx, err := client.NewTransaction(ctx)
		if err != nil {
			break
		}

		var c Counter
		if err := tx.Get(key, &c); err != nil && err != datastore.ErrNoSuchEntity {
			break
		}
		c.Count++
		if _, err := tx.Put(key, &c); err != nil {
			break
		}

		// Attempt to commit the transaction. If there's a conflict, try again.
		if _, err := tx.Commit(); err != datastore.ErrConcurrentTransaction {
			break
		}
	}

}

Output:
Example (GetOrCreate) 
package main

import (
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	key := datastore.NewKey(ctx, "Task", "sampletask", 0, nil)
	// [START transactional_get_or_create]
	_, err := client.RunInTransaction(ctx, func(tx *datastore.Transaction) error {
		var task Task
		if err := tx.Get(key, &task); err != datastore.ErrNoSuchEntity {
			return err
		}
		_, err := tx.Put(key, &Task{
			Type:        "Personal",
			Done:        false,
			Priority:    4,
			Description: "Learn Cloud Datastore",
		})
		return err
	})
	// [END transactional_get_or_create]
	_ = err // Check error.
}

Output:
Example (Insert) 
package main

import (
	"log"
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	task := &Task{} // Populated with appropriate data.
	taskKey := datastore.NewKey(ctx, "Task", "sampleTask", 0, nil)
	// [START insert]
	tx, err := client.NewTransaction(ctx)
	if err != nil {
		log.Fatalf("client.NewTransaction: %v", err)
	}
	// We first check that there is no entity stored with the given key.
	if err := tx.Get(taskKey, nil); err != datastore.ErrNoSuchEntity {
		log.Fatalf("tx.Get returned err %v, want ErrNoSuchEntity", err)
	}
	if _, err := tx.Put(taskKey, task); err != nil {
		log.Fatalf("tx.Put: %v", err)
	}
	if _, err := tx.Commit(); err != nil {
		log.Fatalf("tx.Commit: %v", err)
	}
	// [END insert]
}

Output:
Example (RunQuery) 
package main

import (
	"log"
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	// [START transactional_single_entity_group_read_only]
	tx, err := client.NewTransaction(ctx)
	if err != nil {
		log.Fatalf("client.NewTransaction: %v", err)
	}
	defer tx.Rollback() // Transaction only used for read.

	ancestor := datastore.NewKey(ctx, "TaskList", "default", 0, nil)
	query := datastore.NewQuery("Task").Ancestor(ancestor).Transaction(tx)
	var tasks []Task
	_, err = client.GetAll(ctx, query, &tasks)
	// [END transactional_single_entity_group_read_only]
	_ = err // Check error.
}

Output:
Example (Update) 
package main

import (
	"log"
	"time"

	"cloud.google.com/go/datastore"
	"golang.org/x/net/context"
)

type Task struct {
	Type            string
	Done            bool
	Priority        int
	Description     string `datastore:",noindex"`
	PercentComplete float64
	Created         time.Time
	Tags            []string
	Collaborators   []string
}

func main() {
	ctx := context.Background()
	client, _ := datastore.NewClient(ctx, "my-proj")
	taskKey := datastore.NewKey(ctx, "Task", "sampleTask", 0, nil)
	// [START update]
	tx, err := client.NewTransaction(ctx)
	if err != nil {
		log.Fatalf("client.NewTransaction: %v", err)
	}
	var task Task
	if err := tx.Get(taskKey, &task); err != nil {
		log.Fatalf("tx.Get: %v", err)
	}
	task.Priority = 5
	if _, err := tx.Put(taskKey, task); err != nil {
		log.Fatalf("tx.Put: %v", err)
	}
	if _, err := tx.Commit(); err != nil {
		log.Fatalf("tx.Commit: %v", err)
	}
	// [END update]
}

Output:

func (*Transaction) Commit 

func (t *Transaction) Commit() (*Commit, error)

Commit applies the enqueued operations atomically.

func (*Transaction) Delete 

func (t *Transaction) Delete(key *Key) error

Delete is the transaction-specific version of the package function Delete. Delete enqueues the deletion of the entity for the given key, to be committed atomically upon calling Commit.

func (*Transaction) DeleteMulti 

func (t *Transaction) DeleteMulti(keys []*Key) error

DeleteMulti is a batch version of Delete.

func (*Transaction) Get 

func (t *Transaction) Get(key *Key, dst interface{}) error

Get is the transaction-specific version of the package function Get. All reads performed during the transaction will come from a single consistent snapshot. Furthermore, if the transaction is set to a serializable isolation level, another transaction cannot concurrently modify the data that is read or modified by this transaction.

func (*Transaction) GetMulti 

func (t *Transaction) GetMulti(keys []*Key, dst interface{}) error

GetMulti is a batch version of Get.

func (*Transaction) Put 

func (t *Transaction) Put(key *Key, src interface{}) (*PendingKey, error)

Put is the transaction-specific version of the package function Put.

Put returns a PendingKey which can be resolved into a Key using the return value from a successful Commit. If key is an incomplete key, the returned pending key will resolve to a unique key generated by the datastore.

func (*Transaction) PutMulti 

func (t *Transaction) PutMulti(keys []*Key, src interface{}) ([]*PendingKey, error)

PutMulti is a batch version of Put. One PendingKey is returned for each element of src in the same order.

func (*Transaction) Rollback 

func (t *Transaction) Rollback() error

Rollback abandons a pending transaction.

type TransactionOption 

type TransactionOption interface {
	// contains filtered or unexported methods
}

A TransactionOption configures the Transaction returned by NewTransaction.

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