Collections of entity elements and relationships may be represented by java.util.Map collections. A Map consists of a key and value.
When using Map elements or relationships, the following rules apply:
The Map key or value may be a basic Java programming language type, an embeddable class, or an entity.
When the Map value is an embeddable class or basic type, use the @ElementCollection annotation.
When the Map value is an entity use the @OneToMany or @ManyToMany annotation.
Only use the Map type on one side of a bidirectional relationship.
If the key type of a Map is a Java programming language basic type, use the javax.persistence.MapKeyColumn annotation to set the column mapping for the key. By default, the name attribute of @MapKeyColumn is of the form RELATIONSHIP FIELD/PROPERTY NAME_KEY. For example, if the referencing relationship field name is image, the default name attribute is IMAGE_KEY.
If the key type of a Map is an entity, use the javax.persistence.MapKeyJoinColumn annotation. If the multiple columns are needed to set the mapping, use the javax.persistence.MapKeyJoinColumns annotation to include multiple @MapKeyJoinColumn annotations. If no @MapKeyJoinColumn is present, the mapping column name is by default set to RELATIONSHIP FIELD/PROPERTY NAME_KEY. For example, if the relationship field name is employee, the default name attribute is EMPLOYEE_KEY.
If Java programming language generic types are not used in the relationship field or property, the key class must be explicitly set using the javax.persistence.MapKeyClass annotation.
If the Map key is the primary key, or a persistent field or property of the entity that is the Map value, use the javax.persistence.MapKey annotation. The @MapKeyClass and @MapKey annotations cannot be used on the same field or property.
If Map value is a Java programming language basic type or an embeddable class, it will be mapped as a collection table in the underlying database. If generic types are not used, the @ElementCollection annotation's targetClass attribute must be set to the type of the Map value.
If the Map value is an entity, and part of a many-to-many or one-to-many unidirectional relationship, it will be mapped as a join table in the underlying database. A unidirectional one-to-many relationship that uses a Map may also be mapped using the @JoinColumn annotation.
If the entity is part of a one-to-many/many-to-one bidirectional relationship, it will be mapped in the table of the entity that represents the value of the Map. If generic types are not used, the targetEntity attribute of the @OneToMany and @ManyToMany annotations must be set to the type of the Map value.