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Published: Jun 30, 2015 License: Apache-2.0, Apache-2.0 Imports: 17 Imported by: 0



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) {
	c := appengine.NewContext(r)

	k := datastore.NewKey(c, "Entity", "stringID", 0, nil)
	e := new(Entity)
	if err := datastore.Get(c, k, e); err != nil {
		http.Error(w, err.Error(), 500)

	old := e.Value
	e.Value = r.URL.Path

	if _, err := datastore.Put(c, k, e); err != nil {
		http.Error(w, err.Error(), 500)

	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.


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 App Engine requests. 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.

Fields (except for []byte) are indexed by default. Strings longer than 500 characters cannot be indexed; fields used to store long strings should be tagged with "noindex". Similarly, ByteStrings longer than 500 bytes cannot be indexed.

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

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(c <-chan Property) error {
	// Load I and J as usual.
	if err := datastore.LoadStruct(x, c); err != nil {
		return err
	// Derive the Sum field.
	x.Sum = x.I + x.J
	return nil

func (x *CustomPropsExample) Save(c chan<- Property) error {
	defer close(c)
	// 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, c)", but is done manually for
	// demonstration purposes.
	c <- datastore.Property{
		Name:  "I",
		Value: int64(x.I),
	c <- datastore.Property{
		Name:  "J",
		Value: int64(x.J),
	return nil

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


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) {
	c := appengine.NewContext(r)
	q := datastore.NewQuery("Widget").
		Filter("Price <", 1000).
	b := new(bytes.Buffer)
	for t := q.Run(c); ; {
		var x Widget
		key, err := t.Next(&x)
		if err == datastore.Done {
		if err != nil {
			serveError(c, w, err)
		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)


RunInTransaction runs a function in a transaction.

Example code:

type Counter struct {
	Count int

func inc(c appengine.Context, key *datastore.Key) (int, error) {
	var x Counter
	if err := datastore.Get(c, key, &x); err != nil && err != datastore.ErrNoSuchEntity {
		return 0, err
	if _, err := datastore.Put(c, key, &x); err != nil {
		return 0, err
	return x.Count, nil

func handle(w http.ResponseWriter, r *http.Request) {
	c := appengine.NewContext(r)
	var count int
	err := datastore.RunInTransaction(c, func(c appengine.Context) error {
		var err1 error
		count, err1 = inc(c, datastore.NewKey(c, "Counter", "singleton", 0, nil))
		return err1
	}, nil)
	if err != nil {
		serveError(c, w, err)
	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	fmt.Fprintf(w, "Count=%d", count)



This section is empty.


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.


func AllocateIDs

func AllocateIDs(c appengine.Context, kind string, parent *Key, n int) (low, high int64, err error)

AllocateIDs returns a range of n integer IDs with the given kind and parent combination. kind cannot be empty; parent may be nil. The IDs in the range returned will not be used by the datastore's automatic ID sequence generator and may be used with NewKey without conflict.

The range is inclusive at the low end and exclusive at the high end. In other words, valid intIDs x satisfy low <= x && x < high.

If no error is returned, low + n == high.

func Delete

func Delete(c appengine.Context, key *Key) error

Delete deletes the entity for the given key.

func DeleteMulti

func DeleteMulti(c appengine.Context, key []*Key) error

DeleteMulti is a batch version of Delete.

func Get

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

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

func GetMulti(c appengine.Context, key []*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.

func LoadStruct

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

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

func RunInTransaction

func RunInTransaction(c appengine.Context, f func(tc appengine.Context) error, opts *TransactionOptions) 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.

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.

Nested transactions are not supported; c may not be a transaction context.


type ByteString

type ByteString []byte

ByteString is a short byte slice (up to 500 bytes) that can be indexed.

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.

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 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.

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(c appengine.Context, kind string, parent *Key) *Key

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

func NewKey

func NewKey(c appengine.Context, kind, stringID string, intID int64, parent *Key) *Key

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

func Put(c appengine.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.

func PutMulti

func PutMulti(c appengine.Context, key []*Key, src interface{}) ([]*Key, error)

PutMulti is a batch version of Put.

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

func (*Key) AppID

func (k *Key) AppID() string

AppID returns the key's application ID.

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

Equal returns whether two keys are equal.

func (*Key) GobDecode

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

func (*Key) GobEncode

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

func (*Key) Incomplete

func (k *Key) Incomplete() bool

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) IntID

func (k *Key) IntID() int64

IntID returns the key's integer ID, which may be 0.

func (*Key) Kind

func (k *Key) Kind() string

Kind returns the key's kind (also known as entity type).

func (*Key) MarshalJSON

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

func (*Key) Namespace

func (k *Key) Namespace() string

Namespace returns the key's namespace.

func (*Key) Parent

func (k *Key) Parent() *Key

Parent returns the key's parent key, which may be nil.

func (*Key) String

func (k *Key) String() string

String returns a string representation of the key.

func (*Key) StringID

func (k *Key) StringID() string

StringID returns the key's string ID (also known as an entity name or key name), which may be "".

func (*Key) UnmarshalJSON

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

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)
	// 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

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 or 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.

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.

func (*Query) Count

func (q *Query) Count(c appengine.Context) (int, error)

Count returns the number of results for the query.

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.

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.

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.

func (*Query) GetAll

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

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.

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.

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.

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 (-).

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.

func (*Query) Run

func (q *Query) Run(c appengine.Context) *Iterator

Run runs the query in the given context.

func (*Query) Start

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

Start returns a derivative query with the given start point.

type TransactionOptions

type TransactionOptions struct {
	// XG is whether the transaction can cross multiple entity groups. In
	// comparison, a single group transaction is one where all datastore keys
	// used have the same root key. Note that cross group transactions do not
	// have the same behavior as single group transactions. In particular, it
	// is much more likely to see partially applied transactions in different
	// entity groups, in global queries.
	// It is valid to set XG to true even if the transaction is within a
	// single entity group.
	XG bool

TransactionOptions are the options for running a transaction.

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