How to trigger foreign key relation without adding a reference @Entity in hibernate?

Question

The following relationship creates a foreign key mapping

@Entity
public class Department {
   @Id
   private String name;

   //some more fields
}

@Entity
public class Employee {
   @Id
   private long id;

   private String name;
   private String designation;

   @ManyToOne
   @JoinColumn(name = "fk_department_id", foreignKey = @ForeignKey(name="fk_department"))
   private Department department;
}

generates:

...CONSTRAINT fk_department FOREIGN KEY (fk_department_id) REFERENCES department (name)

Question: how can I trigger this constraint creation in hibernate without having to create the Department entity?

Eg just adding the foreign key @Id field without an explicit entity reference. But still trigger the fk on initial creation. The following is of course invalid:

   @ManyToOne
   @JoinColumn(name = "fk_department_id", foreignKey = @ForeignKey(name="fk_department"))
   private String department;

You get the intention. Is that possible?

(sidenote: I'm not interested in creating that foreign key link by startup ddl/sql statements).

So you want to create the schema from your entities, but you don't want to write your entities to match the schema? — Kayaman
– Kayaman, Commented Jul 19, 2018 at 11:12
Exactly, in this special case for that foreign table entity. — membersound
– membersound, Commented Jul 19, 2018 at 11:15

membersound · Accepted Answer · 2018-07-19 13:11:48Z

3

You'll have to drop @ManyToOne at least, since that's for entities.

The following should work by overriding the column definition to include the foreign key while creating it

@Column(name = "department_id", columnDefinition = "VARCHAR(255), foreign key (department_id) REFERENCES department(name)")
private String department;

Now there's only a column and a constraint defined, but no relation (as far as Hibernate knows) defined between entities.

Essentially copied from Hibernate and JPA: how to make a foreign key constraint on a String but that was darn hard to find, so I'm not just going to close this as a duplicate! ;)

edited Jul 19, 2018 at 13:11

membersound

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answered Jul 19, 2018 at 11:26

Kayaman

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5 Comments

membersound Over a year ago

Unfortunately as a result of this, no foreign key relation is created by hibernate!

Kayaman Over a year ago

@membersound how about this?

membersound Over a year ago

Unfortunately I'm using mysql, and that does not support inline foreign keys directly. To quote the docs:

MySQL does not recognize or support “inline REFERENCES specifications” (as defined in the SQL standard) where the references are defined as part of the column specification

. Therefore the column definition would have to be as follows:

@Column(name = "department_id", columnDefinition = "varchar(255), foreign key (department_id) REFERENCES department (name)")

. I tried this, it worked. BUT it feels somehow very very hacky...

Kayaman Over a year ago

Well your requirements are hacky. You've already said in your question that you don't want to use either of the 2 clean ways.

membersound Over a year ago

Ok you're probably right. So if there is no "cleaner" way this is probably the answer. Glad it works at all!

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