JPA @Query to count how many relations each object has

Question

I've been trying to get a query inside a join table for a many to many relation working. The query was meant to count how many users follow a specific game. The entity itself is very simple, looks like this:

@Entity
@Table(name = "followed_users_games", uniqueConstraints = {
        @UniqueConstraint(columnNames = "followed_id")
})
public class FollowedEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    @Column(name = "followed_id", unique = true, nullable = false)
    private Integer followedId;

    @ManyToOne
    @JoinColumn(name = "game_id")
    private GameEntity games;

    @ManyToOne
    @JoinColumn(name = "user_id")
    private UserEntity users;

    @Column(name = "notify")
    @NonNull private Boolean notify;
}

And the query I've been trying to get running looks like so

    @Query("select f.gameId, count(f) as usercount from FollowedEntity f group by f.games.gameId order by usercount desc")
    List<GameEntity> findMostFollowed(Pageable pageable);

I have tested the query on my database itself, and it seems to be working fine. However my application returns an error as such:

org.postgresql.util.PSQLException: ERROR: column "gameentity1_.game_id" must appear in the GROUP BY clause or be used in an aggregate function

Any help would be appreciated.

Answer 1

It looks like you have to use join in your query like

@Query(value = "SELECT g.gameId, COUNT(g) as usercount FROM FollowedEntity f JOIN f.games g GROUP By g.gameId ORDER BY usercount DESC")
   List<GameEntity> findMostFollowed(Pageable pageable);

Answer 2

1. You try to map a pair (game_id, count) to the whole GameEntity that has a different structure, that's why sql query that is generated is not what you expect.

2. What can probably help you is mapping your query result to DTO .

3. Every time when you have an issue like this I would recommend to have a look at SQL query that JPA is generating. See, eg, https://www.baeldung.com/sql-logging-spring-boot