Allow entity views to contain subviews with custom correlation

[beikov]

The idea is to allow an attribute on an entity view to be computed by a subquery. The subquery would be executed for every entity view object created.
This is also kind of related to #52 since we might want to specify whether the subquery should do batching or so.

[beikov]

An entity view could contain a subview and the mapping in the main entity view would contain the correlation predicate. Something like the following.

@EntityView(Document.class)
public interface DocumentView {
//...
@MappingSubview(DocumentPermissionCorrelator.class)
public List<PermissionView> getPermissions();
//..
}

public class DocumentPermissionCorrelator implements SubviewCorrelator {

public void applyCorrelation(BaseQueryBuilder<T, ?> queryBuilder, String correlatedAlias);
  queryBuilder.from(SomeEntity.class, "someEntity");
  queryBuilder.where("someEntity.documentId").eqExpression("OUTER(id)");
  queryBuilder.where("someEntity.groupId").eqExpression(correlatedAlias + ".groupId");
}

}

@EntityView(Permission.class)
public interface PermissionView {
//...
}

Maybe the BaseQueryBuilder is an overkill in this case and we should provide a dedicated interface consisting of just FromBuilder and WhereBuilder or parts of them in a different form. This could allow several strategies for fetching the the subview like proposed in #52

• JOIN
• SUBQUERY
• SUBSELECT

To get LEFT JOIN semantics in case of JOIN fetching requires to rewrite some of the predicates because in JPA2-means it can only do a CROSS JOIN to implement this feature.
Another possible implementation would be to rely on the proprietary extensions of

• Hibernate: http://in.relation.to/2016/02/10/hibernate-orm-510-final-release/
• EclipseLink: http://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Querying/JPQL#ON

which would require some extensions to the core parts.

[beikov]

JOIN requires #222

[beikov]

Note that batching with SUBQUERY strategy requires #181 and entity joins from #222