[hibernate-dev] Extended KEY expression support

[Steve Ebersole]

FWIW = for what it is worth.

TBH, to me it is not worth much ;)

I don't do things "just because EclipseLink does it".


On Fri, Jan 27, 2017 at 2:33 PM Christian Beikov <christian.beikov at gmail.com>
wrote:

> Fwiw EclipseLink supports both syntaxes "JOIN KEY(m) k" and also "JOIN
> KEY(m).association".
>
> Am 27.01.2017 um 20:25 schrieb Christian Beikov:
> > I have a little proposal for supporting the use of a KEY expression in
> > the FROM clause and I'd like to hear your opinions on that.
> > Unfortunately the JPA spec does not support that, but since a key of a
> > java.util.Map mapping may be an entity type, we need to specify how
> > one can "join" that key explicitly.
> >
> > Right now(pre HHH-10537), when joining a java.util.Map mapping, an
> > inner join is generated for the map key entity type. In my fix for
> > HHH-10537 I changed the behavior to respect/inherit the join type of
> > the collection join.
> > The problem is, that one can't further join any attributes on that
> > key, because there is no syntax in the HQL or the JPA spec that allows
> > to do that.
> >
> > We need to decide (1) whether we always want to join the entity key or
> > require the user to do that specifically via e.g. something like "JOIN
> > alias.map m JOIN KEY(m) k"
> > and also (2) how the syntax for joining further attributes should look
> > like. If we decide to not allow the "JOIN KEY(m)" syntax for (1) we
> > have to support something like "JOIN KEY(m).association", otherwise we
> > can just use the alias like for normal joins "JOIN k.association".
> > Either way, we have to change the grammar but I'd rather like to
> > support/implement the map key joining syntax like "JOIN KEY(m) k" for
> > (1). A further change to that would be to not generate the implicit
> > key table join anymore but require the user to do the join explicitly.
> > Since that would break backwards compatibility, I'd like to make that
> > behavior configurable and of course, by default it will generate the
> > implicit key join to maintain backwards compatibility. I also propose
> > to switch the default in 6.0 so that the join is not generate anymore.
> > The usage in the JPA Criteria API will unfortunately require a cast
> > since the return type of
> > javax.persistence.metamodel.MapAttribute#key() is
> > javax.persistence.metamodel.Path instead of
> > javax.persistence.metamodel.SingularAttribute but essentially the same
> > functionality is available to a user out of the box.
> > Specifying a custom join for a key would look like this in the
> > Criteria API
> >
> > MapAttribute<Entity, MapKeyEntity, MapValueEntity> mapAttribute = ...
> > Join<Entity, MapKeyEntity> keyEntity =
> > mapAttribute.join((SingularAttribute<? super Entity, ? extends
> > MapKeyEntity>) mapAttribute.key(), JoinType.LEFT);
> > keyEntity.join(...)
> >
> > So the questions again.
> >  1. Do you all agree that this is important and should be done?
> >  2. Agree to not generate implicit joins for keys in future versions?
> >  3. Allow joining the key in a separate join?
> >  4. Allow further joins on a key?
> >  5. Happy with how it can be done in JPA Criteria?
> >
> > In addition to that, it would be nice if anyone could make someone
> > from the JPA EG aware of this.
> > From a JPQL BNF point of view, I'd propose the following changes
> >
> > from
> >
> > join_single_valued_path_expression::=
> >
> identification_variable.{single_valued_embeddable_object_field.}*single_valued_object_field
> >
> >
> > to
> >
> > join_single_valued_path_expression::=
> >
> identification_variable.{single_valued_embeddable_object_field.}*single_valued_object_field
> > |
> >     map_field_identification_variable
> >
> > Regards,
> > Christian
>
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