"many-to-many" with association entity using annotations

gistfile1.java

@Entity
public class Membership implements IMembership {
        @EmbeddedId
        private MembershipKey id;
}
 
@Embeddable
public final class MembershipKey implements IMembershipKey, Serializable {     
        @ManyToOne
        private User user;
        @ManyToOne
        private Grid grid;
}