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 lecacy 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() { ... }
    ...
}

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() { ... }
    ...
}

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