Should the id field of a JPA entity be considered in equals and hashCode?

Question

I hit a problem when writing tests for a database application using JPA2 and EclipseLink:

I add some entity to a database, retrieve it later and want to compare it to an

Answer 1

Relying on database generated Ids in your equals and hashCode implementation is not advisable. You ought to rely on the truly unique/semi-unique attributes of your classes in checking for equality, and in generating the hashcode values. The Hibernate documentation has an extensive page that discusses this, and the facts therein are applicable to more or less every JPA provider.

The underlying reason for using business keys over database generated values in your equals and hashCode implementation is that the JPA provider must actually issue a SELECT after persisting the entity in the database. If you compare objects using the database generated Ids, then you will end up having an equality test that fails in the following scenarios:

• If E1 and E2 are entities of class E (that verifies equality using database generated Ids), then if E1 and E2 will be equal if they haven't been stored in the database yet. This is not what you want, especially if want to store E1 and E2 in some Set before persistence. This is worse if the attributes of E1 and E2 possess different values; the equals implementation would prevent two significantly different entities from being added to a Set, and the hashCode implementation will give you a O(n) lookup time when entities are looked up from a HashMap using the primary key.
• If E1 is a managed entity that has been persisted, and E2 is an entity that has not been persisted, then the equality test would deem that E1 != E2 in the scenario where all the attribute values of E1 and E2 (except for the Ids) are similar. Again, this is probably not what you want, especially if you want to avoid duplicate entities in the database that differ only in their database generated Ids.

The equals and hashCode implementations therefore ought to use business keys, in order to exhibit consistent behavior for both persisted and unpersisted entities.