SecondaryKey (Oracle - Berkeley DB Java API)
Date: 2012-05-11

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## com.sleepycat.persist.model
Annotation Type SecondaryKey

    @Documented
    @Retention(value=RUNTIME)
    @Target(value=FIELD)
    public @interface SecondaryKey

Indicates a secondary key field of an entity class. The value of the secondary key field is a unique or non-unique identifier for the entity and is accessed via a `SecondaryIndex`.

`SecondaryKey` may appear on any number of fields in an entity class, subclasses and superclasses. For a secondary key field in the entity class or one of its superclasses, all entity instances will be indexed by that field (if it is non-null). For a secondary key field in an entity subclass, only instances of that subclass will be indexed by that field (if it is non-null).

If a secondary key field is null, the entity will not be indexed by that key. In other words, the entity cannot be queried by that secondary key nor can the entity be found by iterating through the secondary index.

For a given entity class and its superclasses and subclasses, no two secondary keys may have the same name. By default, the field name identifies the secondary key and the secondary index for a given entity class. `name()` may be specified to override this default.

Using `relate()`, instances of the entity class are related to secondary keys in a many-to-one, one-to-many, many-to-many, or one-to-one relationship. This required property specifies the cardinality of each side of the relationship.

A secondary key may optionally be used to form a relationship with instances of another entity class using `relatedEntity()` and `onRelatedEntityDelete()`. This establishes foreign key constraints for the secondary key.

The secondary key field type must be a Set, Collection or array type when a x-to-many relationship is used or a singular type when an x-to-one relationship is used; see `relate()`.

The field type (or element type, when a Set, Collection or array type is used) of a secondary key field must follow the same rules as for a primary key type. The key sort order is also the same.

For a secondary key field with a collection type, a type parameter must be used to specify the element type. For example `Collection<String>` is allowed but `Collection` is not.

Required Element Summary
`Relationship` | `relate` Defines the relationship between instances of the entity class and the secondary keys.

Optional Element Summary
`String` | `name` Specifies the name of the key in order to use a name that is different than the field name.
`DeleteAction` | `onRelatedEntityDelete` Specifies the action to take when a related entity is deleted having a primary key value that exists as a secondary key value for this entity.
`Class` | `relatedEntity` Specifies the entity to which this entity is related, for establishing foreign key constraints.

Element Detail

### relate

    public abstract Relationship relate

  Defines the relationship between instances of the entity class and the secondary keys.

  The table below summarizes how to create all four variations of relationships.

  Relationship | Field type | Key type | Example
  --- | --- | --- | ---
  `Relationship.ONE_TO_ONE` | Singular | Unique | A person record with a unique social security number key.
  `Relationship.MANY_TO_ONE` | Singular | Duplicates | A person record with a non-unique employer key.
  `Relationship.ONE_TO_MANY` | Set/Collection/array | Unique | A person record with multiple unique email address keys.
  `Relationship.MANY_TO_MANY` | Set/Collection/array | Duplicates | A person record with multiple non-unique organization keys.

  For a many-to-x relationship, the secondary index will have non-unique keys; in other words, duplicates will be allowed. Conversely, for one-to-x relationship, the secondary index will have unique keys.

  For a x-to-one relationship, the secondary key field is singular; in other words, it may not be a Set, Collection or array type. Conversely, for a x-to-many relationship, the secondary key field must be a Set, Collection or array type. A collection type is any implementation of `Collection`.

  For a x-to-many relationship, the field type should normally be `Set` (or a subtype of this interface). This accurately expresses the fact that an Entity may not have two identical secondary keys. For flexibility, a `Collection` (or a subtype of this interface) or an array type may also be used. In that case, any duplicate key values in the Collection or array are ignored.

### relatedEntity

    public abstract Class relatedEntity

  Specifies the entity to which this entity is related, for establishing foreign key constraints. Values of this secondary key will be constrained to the set of primary key values for the given entity class.

  The given class must be an entity class. This class is called the related entity or foreign entity.

  When a related entity class is specified, a check (foreign key constraint) is made every time a new secondary key value is stored for this entity, and every time a related entity is deleted.

  Whenever a new secondary key value is stored for this entity, it is checked to ensure it exists as a primary key value of the related entity. If it does not, an exception is thrown by the `PrimaryIndex` `put` method.

  Whenever a related entity is deleted and its primary key value exists as a secondary key value for this entity, the action is taken that is specified using the `onRelatedEntityDelete()` property.

  Together, these two checks guarantee that a secondary key value for this entity will always exist as a primary key value for the related entity. Note, however, that a transactional store must be configured to guarantee this to be true in the face of a crash; see `StoreConfig.setTransactional(boolean)`.

  Default:
    void.class

### onRelatedEntityDelete

    public abstract DeleteAction onRelatedEntityDelete

  Specifies the action to take when a related entity is deleted having a primary key value that exists as a secondary key value for this entity.

  Note: This property only applies when `relatedEntity()` is specified to define the related entity.

  The default action, `ABORT`, means that an exception is thrown in order to abort the current transaction.

  If `CASCADE` is specified, then this entity will be deleted also. This in turn could trigger further deletions, causing a cascading effect.

  If `NULLIFY` is specified, then the secondary key in this entity is set to null and this entity is updated. If the key field type is singular, the field value is set to null; therefore, to specify `NULLIFY` for a singular key field type, a primitive wrapper type must be used instead of a primitive type. If the key field type is an array or collection type, the key is deleted from the array (the array is resized) or from the collection (using `Collection.remove`).

  Default:
    com.sleepycat.persist.model.DeleteAction.ABORT

### name

    public abstract String name

  Specifies the name of the key in order to use a name that is different than the field name.

  This is convenient when prefixes or suffices are used on field names. For example:

        class Person {
            @SecondaryKey(relate=MANY_TO_ONE, relatedEntity=Person.class, name="parentSsn")
            String m_parentSsn;
        }

  It can also be used to uniquely name a key when multiple secondary keys for a single entity class have the same field name. For example, an entity class and its subclass may both have a field named 'date', and both fields are used as secondary keys. The `name` property can be specified for one or both fields to give each key a unique name.

  Default:
    ""

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