cannot simultaneously fetch multiple bags - 1:N 다중 JOIN 도중 오류

Entity2 와 Entity3를 List로 받으려고 시도 했을 때 발생한 오류 - SET 바꿔서 해결(?)

양방향, 단방향, 1:N, N:1, N:M 항상 신경써야함..

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Entity 1

@Getter @Setter
@Entity(name = "a_table")
public class ATable {
	
	@OneToMany
	@JoinColumn(name = "user_id")
	private Set<B> b;
	
	@OneToMany
	@JoinColumn(name="user_id")
	private Set<C> c;
    
    @Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name="id", updatable = false, nullable = false, insertable=false)
	private Long id;
    
    @Column(name="user_id")
	private Long userId;
}

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Entity 2

@Getter @Setter
@Entity(name = "b_table")
public class BTable {
	
	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name="id", updatable = false, nullable = false, insertable=false)
	private Long id;
	
	@Column(name="user_id")
	private Long userId;
	
}

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Entity 3

@Getter @Setter
@Entity(name = "c_table")
public class CTable {
	
	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name="id", updatable = false, nullable = false, insertable=false)
	private Long id;
	
	@Column(name="user_id")
	private Long userId;
    
}

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ATableRepository

@Repository
@Component
public interface ATableRepository extends JpaRepository<ATable, Long>{
	
	@EntityGraph(attributePaths={"b", "c"})
	@Query(value = "SELECT a FROM a_table AS c 
    				LEFT JOIN FETCH b_table b ON a.userId = b.userId
    				LEFT JOIN FETCH c_table c ON a.userId = c.userId
    				GROUP BY a.userId")
	List<ATable> findAll(); //이 부분은 적당히 수정
	
	
}

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