Currently, named queries, entity graphs, and SQL result set mappings are identified by stringly-typed keys.
It would be better if the metamodel generator would generate a typesafe element that identifies each of these things defined within a certain persistence unit. The reference can in principle even encode the result type of the query.
Thus, you would be able to write:
{{TypedQuery<Book> q = book session.createNamedQuery(NamedQueries_.bookWithAuthor);}}
Or:
{{TypedQuery<Book> q = book session.createNativeQuery(sqlQuery, ResultSetMappings_.bookWithAuthor);}}
Or:
{{Book book = session.find(Book.class, EntityGraphs_.bookWithAuthor);}}
Or whatever, and have that all be completely typesafe.
Now, a challenge for this is how to determine the "return type" for each sort of thing.
* For ` {{ @SqlResultSetMapping ` }} and ` {{ @NamedNativeQuery ` }} , it's explicit in the annotation.
* For ` {{ @NamedEntityGraph ` }} , it could reasonably be inferred from the entity class on which the annotation occurs.
* For ` {{ @NamedQuery ` }} we have a problem.
Now, the query-validator project shows how it's in-principle possible to typecheck a named JPA query at compile time, however, that approach is currently limited to Java 8.
There do seem to be some clever ways to infer the result type of a ` {{ @NamedQuery ` }} in specific cases, without typechecking the entire query. For example:
* If there's more than one element in the ` {{ select ` }} , it's totally trivial (must be ` {{ Object \ [] ` }} ).
* If there's no ` {{ select ` }} clause at all, the problem is also pretty easy: either you've got a ` {{ from ` }} with no ` {{ join ` }} , in which case the thing after ` {{ from ` }} is the name of the entity, or you have a ` {{ from ` }} and a ` {{ join ` }} , in which case you're back to ` {{ Object \ [] ` }} .
However, for the specific case of exactly one element in the ` {{ select }} ` clause, we have a problem.
So one option would be to add a result type member to the Hibernate ` {{ @NamedQuery ` }} annotation, so that the user can specify it explicitly (defaulting to ` {{ Object ` }} , of course).
Ultimately this would be a great feature to contribute back to the JPA spec. |
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