How to create join table with JPA annotations?

Question

I need to create a join table in my database using JPA annotations so the result will be this:

So far I just implemented 2 entities:

@Entity
@Table(name="USERS", schema="ADMIN")
public class User implements Serializable {

    private static final long serialVersionUID = -1244856316278032177L;
    @Id 
    @Column(nullable = false)
    private String userid;  
    
    @Column(nullable = false)
    private String password;

    public String getUserid() {
        return userid;
    }

    public void setUserid(String userid) {
        this.userid = userid;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
    
}

---

@Entity
@Table(name="GROUPS", schema="ADMIN")
public class Group implements Serializable {

    private static final long serialVersionUID = -7274308564659753174L;
    @Id
    @Column(nullable = false)
    private String groupid;
    
    public String getGroupid() {
        return groupid;
    }
    public void setGroupid(String groupid) {
        this.groupid = groupid;
    }
}

Should i create another entity called USER_GROUP or i can just add some annotations, so the join table will be created automatically when i run create tables from entities(ORM)?

How should i annotate my entities to achieve the same as in the image?

Answer 1

You definitely shouldn't create User_Group entity as it's more the underlying database representation than the object oriented one.

You can achieve the join table by defining something like:

@Entity
@Table(name="USERS", schema="ADMIN")
public class User implements Serializable {
//...

@ManyToOne
@JoinTable(name="USER_GROUP")
Group group;

---

@Entity
@Table(name="GROUPS", schema="ADMIN")
public class Group implements Serializable {
//...

@OneToMany(mappedBy="group")
Set<User> users;

Edit: If you want to explicitly set the names of the columns you could use @JoinColumn elements as shown below:

@ManyToOne
@JoinTable(name="USER_GROUP",
    joinColumns = @JoinColumn(name = "userid", 
                              referencedColumnName = "userid"), 
    inverseJoinColumns = @JoinColumn(name = "groupid", 
                              referencedColumnName = "groupid"))
Group group;

Answer 2

I would implement it this way:

@Entity
@Table(name="GROUPS", schema="ADMIN")
public class Group implements Serializable {
  @OneToMany
  @JoinTable(name = "USER_GROUP",
            joinColumns = @JoinColumn(name = "groupid"),
            inverseJoinColumns = @JoinColumn(name = "userid"))
  private List<User> users;
}

Solution suggested by @PedroKowalski should work too, but then you'll have to keep a reference to Group entity in your User entity which is not always possible.

Answer 3

To have the same annotations like in your diagram you can do this in your User class:

@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "USER_GROUP", 
           joinColumns = { @JoinColumn(name = "userid") }, 
           inverseJoinColumns = { @JoinColumn(name = "groupid") })
private List<Group> grups;

in your group class

@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "USER_GROUP", 
           joinColumns = { @JoinColumn(name = "groupid") }, 
           inverseJoinColumns = { @JoinColumn(name = "userid") })
private List<User> users;

Answer 4

I'm wondering what is the point to create a Join Table in this way, considering that we can't access directly for queries? JPA doesn't allow to make queries directly to the Join Table, so if the user want to do an operation on USER_GROUP, he has to creare a normal join query between users and groups; due to this, the join table USER_GROUP is useless.