[FIXED] How to insert data in ManyToMany Rest Api

Issue

I can not find the way to insert data using ManyToMany in spring boot. Can anyone please suggest me how to save data using M-To-M in spring boot and hibernate.

Here down is my code.

Entity

@Entity
@Table(name = "user_master")
public class Users {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer user_id;

    private String name;

    @JsonManagedReference
    @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinTable(name = "users_roles", joinColumns = { @JoinColumn(name = "user_id") }, inverseJoinColumns = {
            @JoinColumn(name = "role_id") })
    private Set<Roles> roles;

    // constructor and getter/setter
}

@Entity
@Table(name = "role_master")
public class Roles {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer role_id;

    private String name;

    @JsonBackReference
    @ManyToMany(cascade = CascadeType.ALL, mappedBy = "roles")
    private Set<Users> users;

    // constructor and getter/setter
}

service

@Override
public Set<Users> addAddressPerson(Set<Users> users) {
    for(User user: users) {
        for(Roles roles: user.getRoles()) {
            roles.getUsers().add(user); // problem is in here
        }
    }
    return users.stream().map(userRepo::save).collect(Collectors.toSet());
}

Postman

[
  {
    "name": "Michael",
    "roles": [
    {
        "name": "Project Manager"
    },
    {
        "name": "Software Developer"
    }
    ]
  }
]

Error

"message": "Cannot invoke "java.util.Set.add(Object)" because the return value of "com.rest.RestApiPojo.Entity.Roles.getUsers()" is null"

Solution

try

@Override
public Set<Users> addAddressPerson(List<Users> users) {
    for(User user: users) {
        for(Roles roles: user.getRoles()) {
            roles.getUsers().add(user);
        }
    }
    return users.stream().map(UserRepo::save).collect(Collectors.toSet());
}

and

@Entity
@Table(name = "user_master")
public class Users {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer user_id;

    private String name;

    @JsonManagedReference
    @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinTable(name = "users_roles", joinColumns = { @JoinColumn(name = "user_id") }, inverseJoinColumns = {
            @JoinColumn(name = "role_id") })
    private Set<Roles> roles = new HashSet<>();

    // constructor and getter/setter
}

@Entity
@Table(name = "role_master")
public class Roles {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer role_id;

    private String name;

    @JsonBackReference
    @ManyToMany(cascade = CascadeType.ALL, mappedBy = "roles")
    private Set<Users> users = new HashSet<>();

    // constructor and getter/setter
} 

Answered By - PaulD
Answer Checked By - Robin (JavaFixing Admin)