| Absence of @DiscriminatorColumn causes exception: Unknown column DTYPE [message #809483] |
Tue, 28 February 2012 18:22  |
Eclipse User |
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Hello,
I have the following design:
As you can see, there's a common base table SecurityIdentities for Users and Groups, which are currently mapped via JOINED inheritance *without using a discriminator*.
Here are the mappings:
@Entity
@Table(name = "SecurityIdentities")
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class SecurityIdentity implements Serializable
{
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Basic(optional = false)
@Column
protected Integer id;
@ManyToOne(fetch = FetchType.LAZY, optional = false)
@JoinColumn(name = "identity_type_name", referencedColumnName = "name")
protected IdentityType identityType;
...
}
Now on some EL expression (doesn't matter) the following SQL query is generated to join Users and SecurityIdentities (which causes a runtime exception BTW):
SELECT
t0.ID,
t0.DTYPE, -- UGH!?
t0.identity_type_name,
t1.ID,
t1.is_enabled,
t1.NAME,
t1.PASSWORD,
t1.person_id
FROM
SecurityIdentities t0,
Users t1
WHERE ((t1.ID = t0.ID) AND (t0.DTYPE = ?))
I thought a @DiscriminatorColumn on an inheritance root class is only *optional* for JOINED inheritance. At least Hibernate doesn't need it. (???)
I then found:
http://stackoverflow.com/questions/2063515/why-eclipselink-is-adding-discriminator-column-for-joined-inheritance-strategy
Does EclipseLink really need a discriminator column for JOINED strategy? If so, could it be considered an enhancement to relax the requirement of a discriminator column in a JOINED inheritance? After all, the discriminator column isn't absolutely necessary as entities belonging together could be determined based on the FK columns/values only.
The policy for the code generator could go something like this:
If JOINED inheritance and @DiscriminatorColumn found, use it to join entities and generate the discriminator (as above), otherwise join via FKs and don't generate the DTYPE column in the SQL.
Opinions/comments welcome, especially from the EclipseLink team.
Thanks
Karsten
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| Re: Absence of @DiscriminatorColumn causes exception: Unknown column DTYPE [message #811173 is a reply to message #810805] |
Thu, 01 March 2012 21:15  |
| Eclipse User |
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James Sutherland wrote on Thu, 01 March 2012 10:29... We cannot simply not use a DiscriminatorColumn if none is set because of backward compatibility, spec compliance, and because it is not the best solution to JOINED inheritance.
Hmm about spec compliance, I'm not so sure. The JPA 2 spec from 11/10/09 says:
Quote:11.1.10 DiscriminatorColumn Annotation
For the SINGLE_TABLE mapping strategy, and typically also for the JOINED strategy, the persistence provider will use a type discriminator column. The DiscriminatorColumn annotation is used to define the discriminator column for the SINGLE_TABLE and JOINED inheritance mapping strategies.
The strategy and the discriminator column are only specified in the root of an entity class hierarchy or subhierarchy in which a different inheritance strategy is applied.[90]
The DiscriminatorColumn annotation can be specified on an entity class (including on an abstract entity class).
If the DiscriminatorColumn annotation is missing, and a discriminator column is required, the name of the discriminator column defaults to "DTYPE" and the discriminator type to STRING.
To me the last sentence only says to generate the missing DTYPE column if the discriminator is required. The question is: is a discriminator ultimately required for JOINED inheritance? I have doubts here and it seems it was rather interpreted that way to ease the implementation of EclipseLink.
Final note: In any case, this forces modeling a database column because some technology higher in the stack requires it.
Karsten
[Updated on: Fri, 02 March 2012 04:36] by Moderator |
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