About primary key

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Manual
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JPA Persistable Types

, however, do not have an identity (primary key) of their own which leads to some limitations (e.g ... and arrays and keys and values in maps) are either null values or instances of persistable types. In
JDO Doc
13

javax.jdo.annotations.PrimaryKey

get methods) Annotation on a member to define it as a primary key member of a class or persistent interface using application identity. Also used to define the primary key columns of a secondary table. Corresponds to the xml element "primary-key". Since: JDO 2.1 String column Name of the column to use
JPA Doc
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javax.persistence.JoinColumn

field of the referencing entity or embeddable class; "_"; the name of the referenced primary key ... : the name of the entity; "_"; the name of the referenced primary key column. Default value ... the collection. Default (only applies if single join column is being used): The same name as the primary key
JPA Doc
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JPA Exceptions

) . For example, if a transaction fails because of an attempt to store a new entity object with a primary key that is already in use by another existing entity object, the nested exception is: Other common
Manual
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Shared (L2) Entity Cache

and primary key: em.find(MyEntity2.class, Long.valueOf(1),      Collections.
Manual
12

Comparison in JPQL and Criteria API

have the same type and the same primary key value. For embeddable objects, e1 = e2 if e1 and e2 have exactly
Manual
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Detached Entity Objects

. same type and primary key). If the EntityManager does not manage such an entity object yet a new
Manual
12

Storing JPA Entity Objects

already contains another entity of the same type with the same primary key, an EntityExistsException
Manual
12

JPA Metamodel API

in a type safe manner. The IdentifiableType adds methods for retrieving information on the primary key and the version attributes and the super type. For example: // Get the super type
JPA Doc
12

javax.persistence.SecondaryTable

. If no primary key join columns are specified, the join columns are assumed to reference the primary key columns of the primary table, and have the same names and types as the referenced primary key columns of the primary table. Example 1: Single secondary table with a single primary key column. @Entity