Should JPA Repositories be created by Table or Object?

Question

What is the best practice for creating JPA Repositories?

Right now I have two tables: Media and Ratings.

To find media that is similar a query has to be made to the Rating table to find the interconnections between the different media. This query then returns a list of Media objects.

Should this query be placed in the Rating repository (as it queries the Rating table), or in the Media repository (as it returns a collection of Media objects with the IDs set)?

I have tried searching for best-practices for this particular use-case but haven't found anything relevant.

Update: The SQL query is defined like this:

@Query(value=[SQL query with several joins],nativeQuery=true)
List<Media> mySQLQuery()

it returns a list of mediaId's which can be returned from the function as Media objects.

Answer 1

I solved it by creating a custom implementation of the repository, implementing a custom interface.

First I had to declare the interface:

public interface CustomQuery {
    public List<Integer>myCustomQuery(int id)
}

Then I implemented it in my custom repository. The name is important, it has to begin with the name of the repository interface it is extending. My repo is named MediaRepository so I named the custom implementation MediaRepositoryImpl:

@Component
public class MediaRepositoryImpl implements CustomQuery {
    @PersistenceContext
    EntityManager manager;

    public List<Integer>myCustomQuery(int id){
        Query q = manager.createNativeQuery(SQL_QUERY_GOES_HERE);
        List<Integer> ids = new ArratList<Integer>();

        @SuppressWarnings("unchecked")
        List<Integer> result  = q.getResultList();
        for(Integer o : result){
            //process the results and add them to the list
        }
        return list;
    }
}

This way, you can do custom native queries while still keeping the regular Repositories clean. This approach is also easier to test.

Answer 2

Well, probably neither.

You see when you implement a Repository and do for example findAll() you will get a List of all Objects from the Entity used in the Repository creation.

  interface MyRepo implements<Media, Long>....

  myRepo.findAll() will return a List of Media objects.

What you are trying to do is out of scope from a Repository as it acts on only that particular Entity with a finit operations on that particular Entity.

Also it seems to me that Media and Ratings are connected with a OneToMany or ManyToMany, then this definitely should go to a separate DAO method.