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2.4.5. Single Association related annotations

By default, when Hibernate cannot resolve the association because the expected associated element is not in database (wrong id on the association column), an exception is raised by Hibernate. This might be inconvenient for legacy and badly maintained schemas. You can ask Hibernate to ignore such elements instead of raising an exception using the @NotFound annotation. This annotation can be used on a @OneToOne (with FK), @ManyToOne, @OneToMany or @ManyToMany association. 
@Entity
public class Child {
    ...
    @ManyToOne
    @NotFound(action=NotFoundAction.IGNORE)
    public Parent getParent() { ... }
    ...
}
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Sometimes you want to delegate to your database the deletion of cascade when a given entity is deleted. 
@Entity
public class Child {
    ...
    @ManyToOne
    @OnDelete(action=OnDeleteAction.CASCADE)
    public Parent getParent() { ... }
    ...
}
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In this case Hibernate generates a cascade delete constraint at the database level.
Foreign key constraints, while generated by Hibernate, have a fairly unreadable name. You can override the constraint name by use @ForeignKey. 
@Entity
public class Child {
    ...
    @ManyToOne
    @ForeignKey(name="FK_PARENT")
    public Parent getParent() { ... }
    ...
}

alter table Child add constraint FK_PARENT foreign key (parent_id) references Parent
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2.4.5.1. Lazy options and fetching modes
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EJB3 comes with the fetch option to define lazy loading and fetching modes, however Hibernate has a much more option set in this area. To fine tune the lazy loading and fetching strategies, some additional annotations have been introduced:
  • @LazyToOne: defines the laziness option on @ManyToOne and @OneToOne associations. LazyToOneOption can be PROXY (ie use a proxy based lazy loading), NO_PROXY (use a bytecode enhancement based lazy loading - note that build time bytecode processing is necessary) and FALSE (association not lazy)
  • @LazyCollection: defines the laziness option on @ManyToMany and @OneToMany associations. LazyCollectionOption can be TRUE (the collection is lazy and will be loaded when its state is accessed), EXTRA (the collection is lazy and all operations will try to avoid the collection loading, this is especially useful for huge collections when loading all the elements is not necessary) and FALSE (association not lazy)
  • @Fetch: defines the fetching strategy used to load the association. FetchMode can be SELECT (a select is triggered when the association needs to be loaded), SUBSELECT (only available for collections, use a subselect strategy - please refers to the Hibernate Reference Documentation for more information) or JOIN (use a SQL JOIN to load the association while loading the owner entity). JOIN overrides any lazy attribute (an association loaded through a JOIN strategy cannot be lazy).
The Hibernate annotations overrides the EJB3 fetching options.
Expand
Table 2.3. Lazy and fetch options equivalent
Annotations Lazy Fetch
@[One|Many]ToOne](fetch=FetchType.LAZY) @LazyToOne(PROXY) @Fetch(SELECT)
@[One|Many]ToOne](fetch=FetchType.EAGER) @LazyToOne(FALSE) @Fetch(JOIN)
@ManyTo[One|Many](fetch=FetchType.LAZY) @LazyCollection(TRUE) @Fetch(SELECT)
@ManyTo[One|Many](fetch=FetchType.EAGER) @LazyCollection(FALSE) @Fetch(JOIN)
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