25.4.2. Associations

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JPA offers four entity association types:

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• @ManyToOne
• @OneToOne
• @OneToMany

• @ManyToMany </div>

  And an @ElementCollection for collections of embeddables.

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  Because object associations can be bidirectional, there are many possible combinations of associations. However, not every possible association type is efficient from a database perspective.

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  The closer the association mapping is to the underlying database relationship, the better it will perform.

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  On the other hand, the more exotic the association mapping, the better the chance of being inefficient.

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  Therefore, the @ManyToOne and the @OneToOne child-side association are best to represent a FOREIGN KEY relationship.

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  The parent-side @OneToOne association requires bytecode enhancement so that the association can be loaded lazily. Otherwise, the parent-side is always fetched even if the association is marked with FetchType.LAZY.

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  For this reason, it’s best to map @OneToOne association using @MapsId so that the PRIMARY KEY is shared between the child and the parent entities. When using @MapsId, the parent-side becomes redundant since the child-entity can be easily fetched using the parent entity identifier.

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  For collections, the association can be either:

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• unidirectional

• bidirectional </div>

  For unidirectional collections, Sets are the best choice because they generate the most efficient SQL statements. Unidirectional Lists are less efficient than a @ManyToOne association.

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  Bidirectional associations are usually a better choice because the @ManyToOne side controls the association.

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  Embeddable collections (`@ElementCollection) are unidirectional associations, hence Sets are the most efficient, followed by ordered Lists, whereas bags (unordered Lists) are the least efficient.

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  The @ManyToMany annotation is rarely a good choice because it treats both sides as unidirectional associations.

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  For this reason, it’s much better to map the link table as depicted in the Bidirectional many-to-many with link entity lifecycle section. Each FOREIGN KEY column will be mapped as a @ManyToOne association. On each parent-side, a bidirectional @OneToMany association is going to map to the aforementioned @ManyToOne relationship in the link entity.

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  Just because you have support for collections, it does not mean that you have to turn any one-to-many database relationship into a collection.

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  Sometimes, a @ManyToOne association is sufficient, and the collection can be simply replaced by an entity query which is easier to paginate or filter.

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