ForeignKey
@Target(allowedTargets = )
@Retention(value = AnnotationRetention.BINARY)
public annotation ForeignKey
Declares a foreign key on another Entity.
Foreign keys allows you to specify constraints across Entities such that SQLite will ensure that the relationship is valid when you modify the database.
When a foreign key constraint is specified, SQLite requires the referenced columns to be part of a unique index in the parent table or the primary key of that table. You must create a unique index in the parent entity that covers the referenced columns (Room will verify this at compile time and print an error if it is missing).
It is also recommended to create an index on the child table to avoid full table scans when the parent table is modified. If a suitable index on the child table is missing, Room will print RoomWarnings.MISSING_INDEX_ON_FOREIGN_KEY_CHILD warning.
A foreign key constraint can be deferred until the transaction is complete. This is useful if you are doing bulk inserts into the database in a single transaction. By default, foreign key constraints are immediate but you can change this value by setting deferred to true. You can also use defer_foreign_keys to defer them depending on your transaction.
Please refer to the SQLite foreign keys documentation for details.
Summary
Nested types |
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@IntDef(value = [1, 2, 3, 4, 5])Constants definition for values that can be used in |
Constants |
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static final int |
CASCADE = 5 |
static final int |
NO_ACTION = 1 |
static final int |
RESTRICT = 2 |
static final int |
SET_DEFAULT = 4 |
static final int |
SET_NULL = 3 |
Public constructors |
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Public methods |
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final @NonNull String[] |
The list of column names in the current |
final boolean |
A foreign key constraint can be deferred until the transaction is complete. |
final @NonNull KClass<@NonNull ?> |
The parent Entity to reference. |
final int |
Action to take when the parent |
final int |
Action to take when the parent |
final @NonNull String[] |
The list of column names in the parent |
Constants
CASCADE
public static final int CASCADE = 5
Possible value for onDelete or onUpdate.
A "CASCADE" action propagates the delete or update operation on the parent key to each dependent child key. For onDelete action, this means that each row in the child entity that was associated with the deleted parent row is also deleted. For an onUpdate action, it means that the values stored in each dependent child key are modified to match the new parent key values.
NO_ACTION
public static final int NO_ACTION = 1
Possible value for onDelete or onUpdate.
When a parent key is modified or deleted from the database, no special action is taken. This means that SQLite will not make any effort to fix the constraint failure, instead, reject the change.
RESTRICT
public static final int RESTRICT = 2
Possible value for onDelete or onUpdate.
The RESTRICT action means that the application is prohibited from deleting (for onDelete) or modifying (for onUpdate) a parent key when there exists one or more child keys mapped to it. The difference between the effect of a RESTRICT action and normal foreign key constraint enforcement is that the RESTRICT action processing happens as soon as the field is updated - not at the end of the current statement as it would with an immediate constraint, or at the end of the current transaction as it would with a deferred constraint.
Even if the foreign key constraint it is attached to is deferred, configuring a RESTRICT action causes SQLite to return an error immediately if a parent key with dependent child keys is deleted or modified.
SET_DEFAULT
public static final int SET_DEFAULT = 4
Possible value for onDelete or onUpdate.
The "SET DEFAULT" actions are similar to SET_NULL, except that each of the child key columns is set to contain the columns default value instead of NULL.
SET_NULL
public static final int SET_NULL = 3
Possible value for onDelete or onUpdate.
If the configured action is "SET NULL", then when a parent key is deleted (for onDelete) or modified (for onUpdate), the child key columns of all rows in the child table that mapped to the parent key are set to contain NULL values.
Public constructors
Public methods
getChildColumns
public final @NonNull String[] getChildColumns()
The list of column names in the current Entity.
Number of columns must match the number of columns specified in parentColumns.
| Returns | |
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@NonNull String[] |
The list of column names in the current Entity. |
getDeferred
public final boolean getDeferred()
A foreign key constraint can be deferred until the transaction is complete. This is useful if you are doing bulk inserts into the database in a single transaction. By default, foreign key constraints are immediate but you can change it by setting this field to true. You can also use defer_foreign_keys PRAGMA to defer them depending on your transaction.
| Returns | |
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boolean |
Whether the foreign key constraint should be deferred until the transaction is complete. Defaults to |
getEntity
public final @NonNull KClass<@NonNull ?> getEntity()
The parent Entity to reference. It must be a class annotated with Entity and referenced in the same database.
getOnDelete
@ForeignKey.Action
public final int getOnDelete()
Action to take when the parent Entity is deleted from the database.
By default, NO_ACTION is used.
| Returns | |
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int |
The action to take when the referenced entity is deleted from the database. |
getOnUpdate
@ForeignKey.Action
public final int getOnUpdate()
Action to take when the parent Entity is updated in the database.
By default, NO_ACTION is used.
| Returns | |
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int |
The action to take when the referenced entity is updated in the database. |
getParentColumns
public final @NonNull String[] getParentColumns()
The list of column names in the parent Entity.
Number of columns must match the number of columns specified in childColumns.
| Returns | |
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@NonNull String[] |
The list of column names in the parent Entity. |
| See also | |
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childColumns |