Hibernate - ManyToOne & Inheritance / JOINED / mappedBy

Question

I have some problems with inheritance mapping. Here my database structure:

And associated entities:

AbstractEntity:

@MappedSuperclass
public abstract class AbstractEntity<ID extends Serializable> implements Serializable {

@Id @GeneratedValue(strategy = IDENTITY)
@Column(unique = true, updatable = false, nullable = false)
private ID id;

public ID getId() {
return id;
}
@SuppressWarnings("unused")
public void setId(ID id) {
this.id = id;
}

UserAcitvity entity:

@Entity @Table(name = "user_activity")
@Inheritance(strategy = JOINED)
@AttributeOverride(name = "id", column = @Column(name = "ua_id"))
public abstract class UserActivity extends AbstractEntity<Long> {

@ManyToOne(cascade = { MERGE, PERSIST }, fetch = LAZY)
@JoinColumn(name = "ua_user_id")
private User user;

...
}

Comment entity:

@Entity @Table(name = "comment")
@PrimaryKeyJoinColumn(name = "cm_id")
public class Comment extends UserActivity {

@ManyToOne(cascade = { MERGE, PERSIST }, fetch = LAZY)
@JoinColumn(name = "cm_question_id")
private Question question;

...
}

Question entity:

@Entity @Table(name = "question")
@PrimaryKeyJoinColumn(name = "qs_id")
public class Question extends UserActivity {

...

@OneToMany(fetch = LAZY, cascade = ALL, mappedBy = "question")
private List<Answer> answers = new ArrayList<>();

@OneToMany(fetch = LAZY, cascade = ALL, mappedBy = "question")
private List<Comment> comments = new ArrayList<>();

...
}

Answer entity:

@Entity @Table(name = "answer")
@PrimaryKeyJoinColumn(name = "asw_id")
public class Answer extends UserActivity {

@ManyToOne(cascade = { MERGE, PERSIST }, fetch = LAZY)
@JoinColumn(name = "asw_question_id")
private Question question;

...
}

and User entity:

@Entity @Table(name = "user")
@AttributeOverride(name = "id", column = @Column(name = "user_id"))
public class User extends AbstractEntity<Long> {

...
@OneToMany(cascade = REMOVE)
private List<Question> questions = new ArrayList<>();

@OneToMany(cascade = REMOVE)
private List<Answer> answers = new ArrayList<>();

@OneToMany(cascade = REMOVE)
private List<Comment> comments = new ArrayList<>();
...
}

Problem:

When I try to save or delete a User I get an exceptions:

org.springframework.dao.InvalidDataAccessResourceUsageException: could not prepare statement; SQL [insert into user_question (user_user_id, questions_qs_id) values (?, ?)]; nested exception is org.hibernate.exception.SQLGrammarException: could not prepare statement

and:

org.hibernate.engine.jdbc.spi.SqlExceptionHelper : 147 = user lacks privilege or object not found: USER_ANSWER

Hibernate is trying to create a table: user_question and user_answer which me do not need.

What I should doing for fixes ?

Solution 1

I don't think you can achieve this by mapping the ManyToOne association to User generically in the UserActivity entity. That's probably too confusing for the JPA provider (Hibernate).

Instead, I think you need to map the association to User in each of the Question, Answer and Comment entities. Yes, I know that would be duplicated code, but it looks like the only way you will then be able to qualify the OneToMany mappings in User using the mappedBy reference.

For instance, your Question entity would have an association defined as:

@ManyToOne(cascade = { MERGE, PERSIST }, fetch = LAZY)
@JoinColumn(name = "ua_user_id")
private User questionUser;

Depending on how clever (or not) Hibernate is about the above association, you may need to specify the table="USER_ACTIVITY" in the JoinColumn annotation.

Then the User would have the OneToMany as:

@OneToMany(mappedBy="questionUser", cascade = REMOVE)
private List<Question> questions = new ArrayList<>();

Similarly for each of Answer and Comment.

Of course, I haven't tried this, so I could be wrong.

Solution 2

It's probably happening because when you set the @OneToMany mapping then the hibernate will create an auxiliary table that will store the id from the entities on the relationship.

In this case you should try the following:

@OneToMany(cascade = REMOVE)
@JoinColumn(name = "answer_id")
private List<Answer> answers = new ArrayList<>();

The @JoinColumn annotation will map the relationship without the creation of the auxiliary table, so it's pretty likely this solution will help you in this situation.

Solution 3

Try this mapping, this should work as you expect according to section 2.2.5.3.1.1 of the documentation:

@Entity
public class User {
    @OneToMany(cascade = REMOVE) 
    @JoinColumn(name="user_fk") //we need to duplicate the physical information
    private List<Question> questions = new ArrayList<>();
        ...
}

@Entity
public class Question {
    @ManyToOne
    @JoinColumn(name="user_fk", insertable=false, updatable=false)
    private User user;
    ...
}

The reason why the auxiliary association is created, is that there is no way for Hibernate to know that the Many side of the relation (for example Question) has a foreign key back to User that corresponds to the exact same relation as User.questions.

The association Question.user could be a completely different association, for example User.questionCreator or User.previousSuccessfulAnswerer.

Just by looking at Question.user, there is no way for Hibernate to know that it's the same association as User.questions.

So without the mappedBy indicating that the relation is the same, or @JoinColumn to indicate that there is no join table (but only a join column), Hibernate will trigger the generic one-to-many association mapping solution that consists in creating an auxiliary mapping table.

The schema misses such association tables, which causes the error that can be solved with the mapping above.