Question

I have set of classes which inherit from a single super class:

   Super
     |
     +------+-------+
     Aaaa   Bbbb    Cccc

Each of the Aaaa,Bbbb,Cccc then should contain method findByTag. The problem is that I can’t manage to define it generally. Following example defines specific findByTag for Aaaa.

public interface AaaaRepository extends SuperRepository<Aaaa> {
    @Query("select distinct a from Aaaa a " +
            "join a.tags t " +
            "join fetch a.locale where t = ?1")
    public List<Event> findByTag(Tag t);
}

Note that the Superclass is @MappedSuperclass and does not have its own table in database.

I would like to use some kind of “Super” in the query which would be replaced in each class by its name.

My second problem is that I don’t know how to force @ElementCollection to be Eagerly fetched. I have to always explicitly say “join fetch” in the query. If it is not fetched, once the transaction is finished, I can’t access those objects, which I did not explicitly fetched. (LazyFetch Exceptions…)

Thanks

Answer 1

Looking at the documentation, custom implementations section, what about this approach:

1. Create an interface that extends repository and has your findByTag method, without annotations.
2. Create an implementation of that class, and in the method implementation you use the JPA criteria. You also need a class field to hold the actual class for the domain object, because generics are erased at compilation time. Then you use that field to build the criteria.

3. Read the documentation to use this implementation as a base class for the repository factory, then Spring Data will build implementations for the other repositories based on this custom one.

   public interface MyRepository<T, ID> extends JpaRepository<T, ID> {        
       public List<Event> findByTag(Tag t);
   }
   
   public class MyRepositoryImpl<T, ID> implements MyRepository<T, ID> {
   
       private Class<T> actualClass; // initialized in the constructor
   
       public List<Event> findByTag(Tag t) {
            // here you build the criteria using actualClass field, and execute it.
       }
   }
   
   public interface AaaaRepository extends MyRepository <Aaaa, Integer> {
       // other methods...
   }
   

Look at “Example 1.16. Custom repository factory bean” of the documentation to create the factory bean.

When Spring instantiates the implementation of AaaaRepository, it will use MyRepositoryImpl as base class.

Will this work for you?