Cannot make @ManyToOne relationship nullable
jboss7.x, jpa-2.0, many-to-one, nullable, postgresql-9.1
Solution
I found the solution to my problem. The way the primary key is defined in entity `Customer` is fine, the problem resides in the foreign key declaration. It should be declared like this:
@ManyToOne
@JoinColumn(columnDefinition="integer", name="customer_id")
private Customer customer;
Indeed, if the attribute `columnDefinition="integer"` is omitted the foreign key will by default be set as the source column: a not-null serial with its own sequence. That is of course not what we want as we just want the to reference the auto-incremented ID, not to create a new one.
Besides, it seems that the attribute `name=customer_id` is also required as I observed when performing some testing. Otherwise the foreign key column will still be set as the source column. This is a strange behavior in my opinion. Comments or additional information to clarify this are welcome!
Finally, the advantage of this solution is that the ID is generated by the database (not by JPA) and thus we do not have to worry about it when inserting data manually or through scripts which often happens in data migration or maintenance.
Problem
I have a many-to-one relationship that I want to be nullable: ``` @ManyToOne(optional = true) @JoinColumn(name = "customer_id", nullable = true) private Customer customer; ``` Unfortunately, JPA keeps setting the column in my database as NOT NULL. Can anyone explain this? Is there a way to make it work? Note that I use JBoss 7, JPA 2.0 with Hibernate as persistence provider and a PostgreSQL 9.1 database. EDIT: I found the cause of my problem. Apparently it is due to the way I defined the primary key in the referenced entity `Customer`: ``` @Entity @Table public class Customer { @Id @GeneratedValue @Column(columnDefinition="serial") private int id; } ``` It seems that using `@Column(columnDefinition="serial")` for the primary key automatically sets the foreign keys referencing it to `NOT NULL` in the database. Is that really the expected behavior when specifying the column type as `serial`? Is there a workaround for enabling nullable foreign keys in this case? Thank you in advance.