Documentation ¶
Overview ¶
Package datastore provides a client for App Engine's datastore service.
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,
- ByteString,
- *Key,
- time.Time (stored with microsecond precision),
- appengine.BlobKey,
- appengine.GeoPoint,
- 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 handle(w http.ResponseWriter, r *http.Request) { ctx := appengine.NewContext(r) k := datastore.NewKey(ctx, "Entity", "stringID", 0, nil) e := new(Entity) if err := datastore.Get(ctx, k, e); err != nil { http.Error(w, err.Error(), 500) return } old := e.Value e.Value = r.URL.Path if _, err := datastore.Put(ctx, k, e); err != nil { http.Error(w, err.Error(), 500) return } w.Header().Set("Content-Type", "text/plain; charset=utf-8") fmt.Fprintf(w, "old=%q\nnew=%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 an appengine.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.
The only valid options are "omitempty" and "noindex".
If the options include "omitempty" and the value of the field is empty, then the field will be omitted on Save. The empty values are false, 0, any nil interface value, and any array, slice, map, or string of length zero. Struct field values will never be empty.
If options include "noindex" then the field will not be indexed. 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.
To use multiple options together, separate them by a comma. The order does not matter.
If the options is "" then the comma may be omitted.
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 nil, 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), }, }, nil }
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 handle(w http.ResponseWriter, r *http.Request) { ctx := appengine.NewContext(r) q := datastore.NewQuery("Widget"). Filter("Price <", 1000). Order("-Price") b := new(bytes.Buffer) for t := q.Run(ctx); ; { var x Widget key, err := t.Next(&x) if err == datastore.Done { break } if err != nil { serveError(ctx, w, err) return } fmt.Fprintf(b, "Key=%v\nWidget=%#v\n\n", key, x) } w.Header().Set("Content-Type", "text/plain; charset=utf-8") io.Copy(w, b) }
Transactions ¶
RunInTransaction runs a function in a transaction.
Example code:
type Counter struct { Count int } func inc(ctx context.Context, key *datastore.Key) (int, error) { var x Counter if err := datastore.Get(ctx, key, &x); err != nil && err != datastore.ErrNoSuchEntity { return 0, err } x.Count++ if _, err := datastore.Put(ctx, key, &x); err != nil { return 0, err } return x.Count, nil } func handle(w http.ResponseWriter, r *http.Request) { ctx := appengine.NewContext(r) var count int err := datastore.RunInTransaction(ctx, func(ctx context.Context) error { var err1 error count, err1 = inc(ctx, datastore.NewKey(ctx, "Counter", "singleton", 0, nil)) return err1 }, nil) if err != nil { serveError(ctx, w, err) return } w.Header().Set("Content-Type", "text/plain; charset=utf-8") fmt.Fprintf(w, "Count=%d", count) }
Metadata ¶
The datastore package provides access to some of App Engine's datastore metadata. This metadata includes information about the entity groups, namespaces, entity kinds, and properties in the datastore, as well as the property representations for each property.
Example code:
func handle(w http.ResponseWriter, r *http.Request) { // Print all the kinds in the datastore, with all the indexed // properties (and their representations) for each. ctx := appengine.NewContext(r) kinds, err := datastore.Kinds(ctx) if err != nil { serveError(ctx, w, err) return } w.Header().Set("Content-Type", "text/plain; charset=utf-8") for _, kind := range kinds { fmt.Fprintf(w, "%s:\n", kind) props, err := datastore.KindProperties(ctx, kind) if err != nil { fmt.Fprintln(w, "\t(unable to retrieve properties)") continue } for p, rep := range props { fmt.Fprintf(w, "\t-%s (%s)\n", p, strings.Join(rep, ", ")) } } }
Index ¶
- Variables
- func Delete(c context.Context, key *Key) error
- func DeleteMulti(c context.Context, key []*Key) error
- func EnableKeyConversion(ctx context.Context)
- func Get(c context.Context, key *Key, dst interface{}) error
- func GetMulti(c context.Context, key []*Key, dst interface{}) (err error)
- func Init(projectID string) error
- func KindProperties(ctx context.Context, kind string) (map[string][]string, error)
- func Kinds(ctx context.Context) ([]string, error)
- func LoadStruct(dst interface{}, p []Property) error
- func Namespaces(ctx context.Context) ([]string, error)
- func RunInTransaction(c context.Context, f func(tc context.Context) error, opts *TransactionOptions) (err error)
- type ByteString
- type Entity
- type ErrFieldMismatch
- type Iterator
- type Key
- func DecodeKey(encoded string) (*Key, error)
- func NewIncompleteKey(c context.Context, kind string, parent *Key) *Key
- func NewKey(c context.Context, kind, stringID string, intID int64, parent *Key) *Key
- func Put(c context.Context, key *Key, src interface{}) (*Key, error)
- func PutMulti(c context.Context, key []*Key, src interface{}) ([]*Key, error)
- func (k *Key) AppID() string
- func (k *Key) Encode() string
- func (k *Key) Equal(o *Key) bool
- func (k *Key) GobDecode(buf []byte) error
- func (k *Key) GobEncode() ([]byte, error)
- func (k *Key) Incomplete() bool
- func (k *Key) IntID() int64
- func (k *Key) Kind() string
- func (k *Key) MarshalJSON() ([]byte, error)
- func (k *Key) Namespace() string
- func (k *Key) Parent() *Key
- func (k *Key) String() string
- func (k *Key) StringID() string
- func (k *Key) UnmarshalJSON(buf []byte) error
- type Property
- type PropertyList
- type PropertyLoadSaver
- type Query
- func (q *Query) Ancestor(ancestor *Key) *Query
- func (q *Query) Count(c context.Context) (int, error)
- func (q *Query) EventualConsistency() *Query
- func (q *Query) Filter(filterStr string, value interface{}) *Query
- func (q *Query) GetAll(c context.Context, dst interface{}) ([]*Key, error)
- func (q *Query) KeysOnly() *Query
- func (q *Query) Limit(limit int) *Query
- func (q *Query) Offset(offset int) *Query
- func (q *Query) Order(fieldName string) *Query
- func (q *Query) Project(fieldNames ...string) *Query
- func (q *Query) Run(c context.Context) *Iterator
- type TransactionOptions
Constants ¶
This section is empty.
Variables ¶
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") )
var Done = errors.New("datastore: query has no more results")
Done is returned when a query iteration has completed.
var ErrConcurrentTransaction = xerrors.New("datastore: concurrent transaction")
ErrConcurrentTransaction is returned when a transaction is rolled back due to a conflict with a concurrent transaction.
Functions ¶
func DeleteMulti ¶
DeleteMulti is a batch version of Delete.
func EnableKeyConversion ¶
EnableKeyConversion enables encoded key compatibility with the Cloud Datastore client library (cloud.google.com/go/datastore). Encoded keys generated by the Cloud Datastore client library will be decoded into App Engine datastore keys.
The context provided must be an App Engine context if running in App Engine first generation runtime. This can be called in the /_ah/start handler. It is safe to call multiple times, and is cheap to call, so can also be inserted as middleware.
Enabling key compatibility does not affect the encoding format used by Key.Encode, it only expands the type of keys that are able to be decoded with DecodeKey.
func Get ¶
Get loads the entity stored for k 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.
func GetMulti ¶
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.
func KindProperties ¶
KindProperties returns all the indexed properties for the given kind. The properties are returned as a map of property names to a slice of the representation types. The representation types for the supported Go property types are:
"INT64": signed integers and time.Time "DOUBLE": float32 and float64 "BOOLEAN": bool "STRING": string, []byte and ByteString "POINT": appengine.GeoPoint "REFERENCE": *Key "USER": (not used in the Go runtime)
func LoadStruct ¶
LoadStruct loads the properties from p to dst. dst must be a struct pointer.
func Namespaces ¶
Namespaces returns all the datastore namespaces.
func RunInTransaction ¶
func RunInTransaction(c context.Context, f func(tc context.Context) error, opts *TransactionOptions) (err error)
RunInTransaction runs f in a transaction. It calls f with a transaction context tc that f should use for all App Engine operations.
If f returns nil, RunInTransaction attempts to commit the transaction, returning nil if it succeeds. If the commit fails due to a conflicting transaction, RunInTransaction retries f, each time with a new transaction context. It gives up and returns ErrConcurrentTransaction after three failed attempts. The number of attempts can be configured by specifying TransactionOptions.Attempts.
If f returns non-nil, then any datastore changes will not be applied and RunInTransaction returns that same error. The function f is not retried.
Note that when f returns, the transaction is not yet committed. Calling code must be careful not to 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. datastore.Get is not idempotent when unmarshaling slice fields.
Nested transactions are not supported; c may not be a transaction context.
Types ¶
type ByteString ¶
type ByteString []byte
ByteString is a short byte slice (up to 1500 bytes) that can be indexed.
type Entity ¶
An Entity is the value type for a nested struct. This type is only used for a Property's Value.
type ErrFieldMismatch ¶
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 Iterator ¶
type Iterator struct {
// contains filtered or unexported fields
}
Iterator is the result of running a query.
func (*Iterator) Next ¶
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 NewIncompleteKey ¶
NewIncompleteKey creates a new incomplete key. kind cannot be empty.
func NewKey ¶
NewKey creates a new key. kind cannot be empty. Either one or both of stringID and intID must be zero. If both are zero, the key returned is incomplete. parent must either be a complete key or nil.
func Put ¶
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.
func PutMulti ¶
PutMulti is a batch version of Put.
src must satisfy the same conditions as the dst argument to GetMulti.
func (*Key) Encode ¶
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) Incomplete ¶
Incomplete returns whether the key does not refer to a stored entity. In particular, whether the key has a zero StringID and a zero IntID.
func (*Key) MarshalJSON ¶
func (*Key) StringID ¶
StringID returns the key's string ID (also known as an entity name or key name), which may be "".
func (*Key) UnmarshalJSON ¶
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 // - ByteString // - *Key // - time.Time // - appengine.BlobKey // - appengine.GeoPoint // - []byte (up to 1 megabyte in length) // - *Entity (representing a nested struct) // This set is smaller than the set of valid struct field types that the // datastore can load and save. A Property Value cannot be a slice (apart // from []byte); use multiple Properties instead. Also, 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. NoIndex bool // Multiple is whether the entity can have multiple properties with // the same name. Even if a particular instance only has one property with // a certain name, Multiple should be true if a struct would best represent // it as a field of type []T instead of type T. Multiple bool }
Property is a name/value pair plus some metadata. A datastore entity's contents are loaded and saved as a sequence of Properties. An entity can have multiple Properties with the same name, provided that p.Multiple is true on all of that entity's Properties with that name.
func SaveStruct ¶
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 or Properties.
type PropertyLoadSaver ¶
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.
func NewQuery ¶
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 ¶
Ancestor returns a derivative query with an ancestor filter. The ancestor should not be nil.
func (*Query) Count ¶
Count returns the number of results for the query.
The running time and number of API calls made by Count scale linearly 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 (*Query) EventualConsistency ¶
EventualConsistency returns a derivative query that returns eventually consistent results. It only has an effect on ancestor queries.
func (*Query) Filter ¶
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.
func (*Query) GetAll ¶
GetAll runs the 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 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 (*Query) KeysOnly ¶
KeysOnly returns a derivative query that yields only keys, not keys and entities. It cannot be used with projection queries.
func (*Query) Limit ¶
Limit returns a derivative query that has a limit on the number of results returned. A negative value means unlimited.
func (*Query) Offset ¶
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 ¶
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 (-).
type TransactionOptions ¶
type TransactionOptions struct { // XG is ignored for Firestore XG bool // Attempts controls the number of retries to perform when commits fail // due to a conflicting transaction. If omitted, it defaults to 3. Attempts int // ReadOnly controls whether the transaction is a read only transaction. // Read only transactions are potentially more efficient. ReadOnly bool }
TransactionOptions are the options for running a transaction.
Source Files ¶
Directories ¶
Path | Synopsis |
---|---|
cloudkey
Package cloudpb is a subset of types and functions, copied from cloud.google.com/go/datastore.
|
Package cloudpb is a subset of types and functions, copied from cloud.google.com/go/datastore. |
cloudpb
Package cloudpb is a subset of protobufs, copied from google.golang.org/genproto/googleapis/datastore/v1.
|
Package cloudpb is a subset of protobufs, copied from google.golang.org/genproto/googleapis/datastore/v1. |