Question:

The scenario is as follows: A user may have many or no comments in the bulletins posted on a system – obviously, a comment belongs to a single user.

The problem is that Hibernate doesn't have any annotations like @OneToZero or anything similar, so how can I do this mapping?

On the user's side ( User class), to list the comments that a user has, I did the following:

@OneToMany(mappedBy = "userID", cascade = CascadeType.ALL)
private List<Comment> comments;

I believe the above mapping is correct. But on the comments side ( Comment class) I don't know what to do to define that a comment can exist in the same way that it can never be realized.

// como mapear aqui? 
@JoinColumn(name = "fk_user")
private User userID;

I even tried doing it with @ManyToOne , but I @ManyToOne a NullPointerException whenever I try to perform any CRUD operation with the user. A simple insert is enough to " crash " the application precisely because the comments are null. But there you have it, a user has no comments when he is registered in the system.

My classes look like this:

@Entity
@Table(name = "users")
public class User implements Serializable {

   @Id
   @GeneratedValue
   private Long id;

   @OneToMany(mappedBy = "userID", cascade = CascadeType.ALL)
   private List<Comment> comments;

   /* ...outros atributos... */
}

@Entity
@Table(name = "comments")
public class Comment implements Serializable {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToOne //ManyToOne gera uma NullPointerException, mas creio que esteja próximo do certo
    @JoinColumn(name = "fk_user")
    private User userID; 

    /* ...outros atributos... */
}

Answer:

Try adding the @NotFound annotation with the attribute (action=NotFoundAction.IGNORE) , leaving:

@Entity
@Table(name = "comments")
public class Comment implements Serializable {

@Id
@GeneratedValue
private Long id;

@NotFound(action=NotFoundAction.IGNORE) //Linha inserida
@JoinColumn(name = "fk_user")
private User userID; 

/* ...outros atributos... */
}

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