如何通过关联链接实体映射两个实体

Question

我需要记录各种业务，他们的城市和他们在每个城市的分支机构。每个企业可能在不同的城市，在每个城市可能有不同的分支机构。

 Business       City
     \          /
  BusinessID CityID    <<join table has primary keys of Business and City tables
          |
        Branch
BranchID BusinessID CityID

Business Table          
1         
2       
3        

City Table
A
B
C

Join Table
Business_City
1         A
1         B
2         A
3         C

Branch Table
Business City   Branch
1          A      I1
1          A      I2
1          B      I6
2          A      I5
3          C      I3

Answer 1

映射应该是这样的：

@Entity
public class Business {
    @Id
    private Long id;

    @OneToMany(mappedBy = "business")
    private Set<Branch> branches = new HashSet<>();
}

@Entity
public class City {
    @Id
    private Long id;

    @OneToMany(mappedBy = "city")
    private Set<Branch> branches = new HashSet<>();
}

@Entity
public class Branch {

    @Id
    private Long id;

    @ManyToOne
    @JoinColumn(name = "business_id")
    private Business business;

    @ManyToOne
    @JoinColumn(name = "city_id")
    private City city;
}

这种中间实体映射比使用@manytomany关联要好，后者可能会生成一些效率较低的SQL查询，并且不允许向链接表添加一些额外的列。