Spring JPA: How to join with fields that don't exist in the entity-CodePudding

We have 3 entities A, B, and C. We are searching in table A with fields within B and C. And the mapping is unidirectional, so B and Collection is not defined in A. We are using Spring Specification, how can we perform join in this case without converting the mapping to bidirectional?

@Entity
public class A {
    @Id
    Long id
    String name
}

@Entity
public class B {
    @Id
    Long id

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "a_id")
    A a;
    
    @Column
    String country;
}

@Entity
public class C {
    @Id
    Long id

  @ManyToOne(fetch = FetchType.LAZY)
  @JoinColumn(name = "a_id")
  private A a;
}

For example, if B is defined in A like that

@Entity
public class A {
    @Id
    Long id
    String name
    
    @OneToOne(mappedBy = "request")
  private B b;
    
}

the join will be like that

 public static Specification<A> countryEquals(String country) {
    return (root, query, builder) -> builder.equal(
        root.join("b", JoinType.LEFT).get("country"), country);
  }

But B is not defined in A, then how to apply the join?

CodePudding user response：

you can try to join it by B itself, you create a new Root and image you add a where b.a_id = a.id to your query or here, you CriteriaBuilder, then you can add more predicate to look for fields in B table. Something like this, somewhat pseudo code cause I don't have my code here atm and write it down by my memory, but I hope it can help

public static Specification<A> countryEquals(String country) {
return (root, query, builder) -> (
    Root<B> b_root = query.from(B.class);
    Predicate b_join = builder.equal(b_root.get("a_id"), root.get("id"))
    Predicate b_country = builder.equal(b_root.get("country"), country) // or like with `%country%`
    return builder.and(other_predicate,...., b_join, b_country)
  )
}