Question

I’ve never seen something like this before

EntityA
@Id
long id

@Column(name ="field1")
String field1
@Column(name ="field2")
String field2

@IdClass(compositePK.class)
EntityB
@Id
String field1
@Id
String field2

@OneToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
EntityA myEntityA

Ok all is ok the tableB need to have the EntityA as foreign key but defined on the id not on field1,field2.

I get error like

Foreign key (FK74D979395D87E41E:EntityA [id])) must have same number of columns as the referenced primary key (EntityA [field1,field2])

I understand that hibernate required that the FK need to be the same as the PK but I can’t made this the entityB PK must stay field1,field2 I can’t change EntityA for the moment.

Answer 1

By what I understand from your code, you’ve got EntityA with simple PK id and EntityB with a composite PK CompositePK. For a @OneToOne association to work, both entities need to share a primary key/have the same PK class (or at least a unique constraint simulating the PK)

It is not possible to use this kind of mapping with this structure, you should either:

• Adapt your data model consistently, so that both EntityA and EntityB share their PK, or there’s a FK from EntityB to EntityA.
• Use a OneToMany instead, even if it will always contain ‘one’ in the ‘many’ side.

See this section on Hibernate Reference for some background: 2.2.5. Mapping entity associations/relationships – One to one, specially the second part talking about FK’s:

There are three cases for one-to-one associations: either the
associated entities share the same primary keys values, a foreign key
is held by one of the entities (note that this FK column in the
database should be constrained unique to simulate one-to-one
multiplicity), or a association table is used to store the link
between the 2 entities (a unique constraint has to be defined on each
fk to ensure the one to one multiplicity).