The value of this argument can be the parent entity itself or its key; you can get the key by calling the parent entity's key method. The following example creates an entity of kind Address and shows two ways of designating an Employee entity as its parent:. Every attempt to create, update, or delete an entity takes place in the context of a transaction.
A single transaction can include any number of such operations. To maintain the consistency of the data, the transaction ensures that all of the operations it contains are applied to Cloud Datastore as a unit or, if any of the operations fails, that none of them are applied. Furthermore, all strongly-consistent reads ancestor queries or gets performed within the same transaction observe a consistent snapshot of the data. As mentioned above, an entity group is a set of entities connected through ancestry to a common root element.
The organization of data into entity groups can limit what transactions can be performed:. In many applications, it is acceptable to use eventual consistency i. In such applications, it is usually a good approach to use a separate entity group for each set of highly related data. For more information, see Structuring for Strong Consistency. The data values associated with an entity consist of one or more properties. Each property has a name and one or more values. A property can have values of more than one type, and two entities can have values of different types for the same property.
Properties can be indexed or unindexed queries that order or filter on a property P will ignore entities where P is unindexed. An entity can have at most 20, indexed properties. Important: We strongly recommend that you do not store a UserProperty , since it includes the email address and the user's unique ID.
If a user changes their email address and you compare their old, stored User to the new User value, they won't match. For text strings and unencoded binary data byte strings , Cloud Datastore supports two value types:. When a query involves a property with values of mixed types, Cloud Datastore uses a deterministic ordering based on the internal representations: Null values Fixed-point numbers Integers Dates and times Ratings Boolean values Byte sequences Byte string Unicode string Blobstore keys Floating-point numbers Geographical points Google Accounts users Cloud Datastore keys.
If the application knows the complete key for an entity or can derive it from its parent key, kind, and identifier , it can use the key to operate directly on the entity.
An application can also obtain an entity's key as a result of a Cloud Datastore query; see the Datastore Queries page for more information. In Python, you create a new entity by constructing an instance of a model class, populating its properties if necessary, and calling its put method to save it to Datastore. If you don't provide a key name, Cloud Datastore will automatically generate a numeric ID for the entity's key:. To retrieve an entity identified by a given key, pass the Key object as an argument to the db. You can generate the Key object using the class method Key. The complete path is a sequence of entities in the ancestor path, with each entity represented by its kind a string followed by its identifier key name or numeric ID :.
Be sure that you have imported the model class for the entity being retrieved. To update an existing entity, modify the attributes of the object, then call its put method. The object data overwrites the existing entity.
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The entire object is sent to Cloud Datastore with every call to put. Given an entity's key, you can delete the entity with the db.
The db. Batch operations do not change your costs. You will be charged for every key in a batched operation, whether or not each key exists. The size of the entities involved in an operation does not affect the cost. If you need to delete a large number of entities, we recommend using Cloud Dataflow to delete entities in bulk.
For the DB interface, empty list writes were historically not allowed at all if the property was dynamic: if you attempted this you got an error. This means that there is no default behavior needing to be preserved for backwards compatibility for DB dynamic properties, and so you can simply write and read the empty list in the dynamic model without any changes. However, for DB static properties, the empty list was written as an omitted property, and this behavior continues by default for backward compatibility. Was this page helpful? Let us know how we did: Send feedback Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.
For details, see our Site Policies. Last updated March 6, Send feedback. These include, among others: Integers Floating-point numbers Strings Dates Binary data For a full list of types, see Properties and value types. The key consists of the following components: The namespace of the entity, which allows for multitenancy The kind of the entity, which categorizes it for the purpose of Cloud Datastore queries An identifier for the individual entity, which can be either a key name string an integer numeric ID An optional ancestor path locating the entity within the Cloud Datastore hierarchy An application can fetch an individual entity from Cloud Datastore using the entity's key, or it can retrieve one or more entities by issuing a query based on the entities' keys or property values.
Kinds and identifiers Each Cloud Datastore entity is of a particular kind, which categorizes the entity for the purpose of queries: for instance, a human resources application might represent each employee at a company with an entity of kind Employee.
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The following example creates an entity of kind Employee , populates its property values, and saves it to Datastore: import datetime from google. It can be assigned in either of two ways: Your application can specify its own key name string for the entity. You can have Cloud Datastore automatically assign the entity an integer numeric ID. The legacy policy creates a sequence of non-consecutive smaller integer IDs. Ancestor paths Entities in Cloud Datastore form a hierarchically structured space similar to the directory structure of a file system.
The complete key identifying the entity consists of a sequence of kind-identifier pairs specifying its ancestor path and terminating with those of the entity itself: [Person:GreatGrandpa, Person:Grandpa, Person:Dad, Person:Me] For a root entity, the ancestor path is empty and the key consists solely of the entity's own kind and identifier: [Person:GreatGrandpa] This concept is illustrated by the following diagram: To designate an entity's parent, use the parent argument to the model class constructor when creating the child entity.
The organization of data into entity groups can limit what transactions can be performed: All the data accessed by a transaction must be contained in at most 25 entity groups. If you want to use queries within a transaction, your data must be organized into entity groups in such a way that you can specify ancestor filters that will match the right data. There is a write throughput limit of about one transaction per second within a single entity group.
This limitation exists because Cloud Datastore performs masterless, synchronous replication of each entity group over a wide geographic area to provide high reliability and fault tolerance. Properties and value types The data values associated with an entity consist of one or more properties.
Entities, Properties, and Keys
Text None Up to 1 megabyte Not indexed Byte string short db. ByteString Byte order Up to bytes Byte string long db. Blob None Up to 1 megabyte Not indexed Date and time datetime. GeoPt By latitude, then longitude Postal address db. Even when there is only 1 field, you still need to annotate the field with the PrimaryKey annotation.
If the entity has a composite primary key, you can use the primaryKeys property of the Entity annotation, as shown in the following code snippet:. By default, Room uses the class name as the database table name. If you want the table to have a different name, set the tableName property of the Entity annotation, as shown in the following code snippet:. Similar to the tableName property, Room uses the field names as the column names in the database. If you want a column to have a different name, add the ColumnInfo annotation to a field, as shown in the following code snippet:.
By default, Room creates a column for each field that's defined in the entity. If an entity has fields that you don't want to persist, you can annotate them using Ignore , as shown in the following code snippet:. In cases where an entity inherits fields from a parent entity, it's usually easier to use the ignoredColumns property of the Entity attribute:. Room supports several types of annotations that make it easier for you to search for details in your database's tables.
Use full-text search unless your app's minSdkVersion is less than To use this capability, available in Room 2. In cases where a table supports content in multiple languages, use the languageId option to specify the column that stores language information for each row:. Room provides several other options for defining FTS-backed entities, including result ordering, tokenizer types, and tables managed as external content.
For more details about these options, see the FtsOptions reference. If your app must support SDK versions that don't allow for using FTS3- or FTS4-table-backed entities, you can still index certain columns in the database to speed up your queries.
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- Marking entities in example utterances;
To add indices to an entity, include the indices property within the Entity annotation, listing the names of the columns that you want to include in the index or composite index. The following code snippet demonstrates this annotation process:. Sometimes, certain fields or groups of fields in a database must be unique.
You can enforce this uniqueness property by setting the unique property of an Index annotation to true. The following code sample prevents a table from having two rows that contain the same set of values for the firstName and lastName columns:. In Room 2.
This support is particularly helpful when two instances of an entity are considered to be equal if their columns contain identical values. When using classes annotated with AutoValue as entities, you can annotate the class's abstract methods using PrimaryKey , ColumnInfo , Embedded , and Relation. When using these annotations, however, you must include the CopyAnnotations annotation each time so that Room can interpret the methods' auto-generated implementations properly.
The following code snippet shows an example of a class annotated with AutoValue that Room recognizes as an entity:. Content and code samples on this page are subject to the licenses described in the Content License. App Basics. Build your first app. App resources. Resource types. App manifest file. App permissions. Device compatibility. Multiple APK support. Adding wearable features to notifications. Creating wearable apps. Creating custom UIs.
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