无法使用 JPA 创建映射实体

Question

A. I have a entity EntityA like below:

Table - EntityA(id long PK, name varchar)

@Entity @Table
EntityA{
    @Id
    long id;

    String name;
}

B. Based on this I want to fetch data in below class through JPA(using unidirectional mapping):

@Entity @Table
EntityMap{

    long id;

    @OneToOne
    EntityA entity;

    @OneToMany
    List<EntityA> mappedEntity;
} 

C. To make it work for now I have created an entity like below:

Table - entity_map(id long pk, source_entity_id long FK-EntityA_id, target_entity_id long FK-EntityA_id)

@Entity @Table
EntityMap{

    @Id
    long id;

    @OneToOne
    @JoinColumn(name = "source_entity_id")
    EntityA sourceEntity;

    @ManyToOne
    @JoinColumn(name = "target_entity_id")
    EntityA targetEntity;
}

This is working as expected but I need entity explained in #B. Any suggestion?

Answer 1

A @OneToMany relationship can be archieved by

@JoinColumn(name = "entity_map_id")
@OneToMany
List<EntityA> mappedEntity;

In your EntityA table you need a column entity_map_id and a foreign key (entity_map_id) references EntityMap(id) constraint.

If you cannot change the EntityA table you need a @JoinTable to perform the mapping:

@JoinTable(name = "JOIN_TABLE", joinColumns = {@JoinColumn(name = "MY_ENTITY_MAP_FK")}, inverseJoinColumns = {@JoinColumn(name = "ENTITY_A_FK")})
@OneToMany
List<EntityA> mappedEntity;

The join table contains two columns ENTITY_A_FK and MY_ENTITY_MAP_FK :

create table JOIN_TABLE(
  ENTITY_A_FK integer not null,
  MY_ENTITY_MAP_FK integer not null
);