JPA insert parent/child results in MySQLIntegrityConstraintViolationException

Question

This has already been asked a number of times, but I don\'t find any good answers so I\'ll ask it again.

I have parent-children unidirectional relation as follows:

Answer 1

Your relationship does not have to be bi-directional. There is some mis-information in the comments here.

You also said that you had added the field "parentId" into the Child entity because you assumed that JPA needs to "know" about the parent field so that it can set the value. The problem is not that JPA does not know about the field, based on the annotations that you have provided. The problem is that you have provided "too much" information about the field, but that information is not internally consistent.

Change your field and annotation in Parent to:

@OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.PERSIST)
@JoinColumn(name = "parent_id")
private List children;

Then remove the "parentId" from the Child entity entirely. You had previously specified a JoinTable annotation. However, what you want is not a JoinTable. A JoinTable would create an additional third table in order to relate the two entities to each other. What you want is only a JoinColumn. Once you add the JoinColumn annotation onto a field that is also annotated with OneToMany, your JPA implementation will know that you are adding a FK into the CHILD table. The problem is that JPA has a CHILD table already defined with a column parent_id.

Think of it that you are giving it two conflicting definitions of both the function of the CHILD table and the parent_id column. In one case, you have told you JPA that it is an entity and the parent_id is simply a value in that entity. In the other, you have told JPA that your CHILD table is not an entity, but is used to create a foreign key relationship between your CHILD and PARENT table. The problem is that your CHILD table already exists. Then when you are persisting your entity, you have told it that the parent_id is explicitly null (not set) but then you have also told it that your parent_id should be updated to set a foreign key reference to the parent table.

I modified your code with the changes I described above, and I also called "persist" instead of "merge".

This resulted in 3 SQL queries

insert into PARENT (ID) values (default)
insert into CHILD (ID) values (default)
update CHILD set parent_id=? where ID=?

This reflects what you want perfectly. The PARENT entry is created. The CHILD entry is created, and then the CHILD record is updated to correctly set the foreign key.

If you instead add the annotation

@OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.PERSIST)
@JoinColumn(name = "parent_id", nullable = false)
private List children;

Then it will run the following query when it inserts the child

insert into CHILD (ID, parent_id) values (default, ?)

thus setting your FK propertly from the very beginning.