jpa 多对多取回数据

Question

I have two entity employee:

public class Employee implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int emp_id;

    private String name;

    private String email;

    private String phone;

    @ManyToMany(
            fetch = FetchType.LAZY, cascade = CascadeType.ALL

    )
    @JoinTable(name = "EMP_PRO_DB", joinColumns = {
            @JoinColumn(name = "empId", referencedColumnName = "emp_id")
    },
    inverseJoinColumns = {
            @JoinColumn(name = "proId", referencedColumnName = "pro_id")
    })
    private Set<Project> projects = new HashSet<>();

}

Project:

public class Project {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int pro_id;

    private String projectName;

    private String projectType;
    @JsonIdentityInfo(
            generator = ObjectIdGenerators.PropertyGenerator.class,
            property = "emp_id"
    )
    @ManyToMany(
            fetch = FetchType.LAZY, cascade = CascadeType.ALL

    )
    @JoinTable(name = "EMP_PRO_DB", joinColumns = {
            @JoinColumn(name = "proId", referencedColumnName = "pro_id")
    },
            inverseJoinColumns = {
                    @JoinColumn(name = "empId", referencedColumnName = "emp_id")
            })
    private Set<Employee> employees = new HashSet<>();
}

When i fetch all data from employee, the json returns me:

which is correct, it can show me the projects that the employee has.

BUT when i fetch all data from project, the json returns me:

Employee 1 has two project, it shows one, employee 2 has one project but its only showing the ID, I will need the details of that employee (at leas the name)

Answer 1

It seems you tried to create a bidirectional relationship between Employee and Project .

But instead you created two conceptually independent relationships, one from Employee to Project and one in the reverse direction, that just happen to write into the same tables. This of course is bound to cause troubles.

The fix is to convert this into a proper bidirectional relationship.

Change the Project side of the relationship to

    @ManyToMany(
            fetch = FetchType.LAZY, cascade = CascadeType.ALL, mappedby="projects"
    )
    private Set<Employee> employees = new HashSet<>();

Note that with this mapping only the Employee side of the relationship will be used while persisting. You'll have to keep the two sides in sync on the Java side yourself.