Insert Into mysql using spring boot Jpa not working

Question

I am trying to persist data into a Mysql db using JpaRepository in spring boot however I keep getting the following error, not sure what am doing wrong:

 java.sql.SQLIntegrityConstraintViolationException: Cannot add or update a child row: a foreign key constraint fails

My Model classes looks like:

@Entity
@Table(name = "cart")
public class Cart {

    @Id
    @Column(name = "id", unique = true, nullable = false)
    private String id;
    @Column(name = "user_id")
    private int userId;

    @OneToMany(fetch = FetchType.LAZY, targetEntity = User.class)
    @JoinColumn(name="user_id", referencedColumnName = "id", insertable = false, updatable = false)
    private List<User> user;

    public List<User> getUser() {
        return user;
    }

    public void setUser(List<User> user) {
        this.user = user;
    }

    public Cart() {
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public int getUserId() {
        return userId;
    }

    public void setUserId(int userId) {
        this.userId = userId;
    }
}


@Entity
@Table(name = "user")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", unique = true, nullable = false, columnDefinition = "integer")
    private Integer id;
    @Column(name = "name")
    private String name;
    @Column(name = "email")
    private String email;
    @Column(name = "phone")
    private String phone;

    public User() {
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }
}

My repository:

    @Repository
public interface CartRepository extends JpaRepository<Cart, String> {


}

What am i doing wrong and is there a better way to persist data?

Update: This is the data am using on postman:

{
    "id": "5beeb524-20fc-11ea-978f-2e728ce88125",
    "user_id": 1
}

However when i log the above data, it returns the id as above but the user_id always returns 0

The id for cart is not auto-generated.

Answer 1

Add a generator for the unique id. For example, for a string the generator could be like :

@Id
@GeneratedValue(generator = "uuid")
@Column(name = "id", unique = true, nullable = false)
private String id;

Find a suitable type and generator for your unique id accordingly. I hope it helps.

EDIT: After seeing the stack trace, this also look somewhat helpful : could not execute statement SQL constraint [id] nested exception is org.hibernate.exception.ConstraintViolationException

Also the issue seems to because the cart.user_id is a foreign key referencing the user.id in user class which is violating some constraints.

Add the following to user model class :

@OneToMany(mappedBy = "user_id", cascade = CascadeType.ALL)
private List<User> users;

And add the following to cart model class :

@ManyToOne
@JoinColumn
private int user_id;