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Diffstat (limited to 'vendor/google.golang.org/appengine/datastore/doc.go')
| -rw-r--r-- | vendor/google.golang.org/appengine/datastore/doc.go | 361 |
1 files changed, 361 insertions, 0 deletions
diff --git a/vendor/google.golang.org/appengine/datastore/doc.go b/vendor/google.golang.org/appengine/datastore/doc.go new file mode 100644 index 000000000..85616cf27 --- /dev/null +++ b/vendor/google.golang.org/appengine/datastore/doc.go @@ -0,0 +1,361 @@ +// Copyright 2011 Google Inc. All rights reserved. +// Use of this source code is governed by the Apache 2.0 +// license that can be found in the LICENSE file. + +/* +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, ", ")) + } + } + } +*/ +package datastore // import "google.golang.org/appengine/datastore" |