java - Hibernate - @ElementCollection - Strange delete/insert behavior

Question

@Entity
public class Person {

    @ElementCollection
    @CollectionTable(name = "PERSON_LOCATIONS", joinColumns = @JoinColumn(name = "PERSON_ID"))
    private List<Location> locations;

    [...]

}

@Embeddable
public class Location {

    [...]

}

Given the following class structure, when I try to add a new location to the list of Person's Locations, it always results in the following SQL queries:

DELETE FROM PERSON_LOCATIONS WHERE PERSON_ID = :idOfPerson

And

A lotsa' inserts into the PERSON_LOCATIONS table

Hibernate (3.5.x / JPA 2) deletes all associated records for the given Person and re-inserts all previous records, plus the new one.

I had the idea that the equals/hashcode method on Location would solve the problem, but it didn't change anything.

Any hints are appreciated!

Answer 1

The problem is somehow explained in the page about ElementCollection of the JPA wikibook:

Primary keys in CollectionTable

The JPA 2.0 specification does not provide a way to define the Id in the Embeddable. However, to delete or update a element of the ElementCollection mapping, some unique key is normally required. Otherwise, on every update the JPA provider would need to delete everything from the CollectionTable for the Entity, and then insert the values back. So, the JPA provider will most likely assume that the combination of all of the fields in the Embeddable are unique, in combination with the foreign key (JoinColunm(s)). This however could be inefficient, or just not feasible if the Embeddable is big, or complex.

And this is exactly (the part in bold) what happens here (Hibernate doesn't generate a primary key for the collection table and has no way to detect what element of the collection changed and will delete the old content from the table to insert the new content).

However, if you define an @OrderColumn (to specify a column used to maintain the persistent order of a list - which would make sense since you're using a List), Hibernate will create a primary key (made of the order column and the join column) and will be able to update the collection table without deleting the whole content.

Something like this (if you want to use the default column name):

@Entity
public class Person {
    ...
    @ElementCollection
    @CollectionTable(name = "PERSON_LOCATIONS", joinColumns = @JoinColumn(name = "PERSON_ID"))
    @OrderColumn
    private List<Location> locations;
    ...
}

References

• JPA 2.0 Specification
  • Section 11.1.12 "ElementCollection Annotation"
  • Section 11.1.39 "OrderColumn Annotation"
• JPA Wikibook
  • Java Persistence/ElementCollection