java - Why @OneToMany does not work with inheritance in Hibernate

Question

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public class Problem {
    @ManyToOne
    private Person person;
}

@Entity
@DiscriminatorValue("UP")
public class UglyProblem extends Problem {}

@Entity
public class Person {
    @OneToMany(mappedBy="person")
    private List< UglyProblem > problems;
}

I think it is pretty clear what I am trying to do. I expect @ManyToOne person to be inherited by UglyProblem class. But there will be an exception saying something like: "There is no such property found in UglyProblem class (mappedBy="person")".

All I found is this. I was not able to find the post by Emmanuel Bernard explaining reasons behind this.

---

Unfortunately, according to the Hibernate documentation "Properties from superclasses not mapped as @MappedSuperclass are ignored."

Well I think this means that if I have these two classes:

public class A {
    private int foo;
}

@Entity
public class B extens A {
}

then field foo will not be mapped for class B. Which makes sense. But if I have something like this:

@Entity
public class Problem {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;

private String name;

public Long getId() {
    return id;
}

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

public String getName() {
    return name;
}

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

@Entity
public class UglyProblem extends Problem {

private int levelOfUgliness;

public int getLevelOfUgliness() {
    return levelOfUgliness;
}

public void setLevelOfUgliness(int levelOfUgliness) {
    this.levelOfUgliness = levelOfUgliness;
}
}

I expect the class UglyProblem to have fileds id and name and both classes to be mapped using same table. (In fact, this is exactly what happens, I have just checked again). I have got this table:

CREATE TABLE "problem" (
    "DTYPE" varchar(31) NOT NULL,
    "id" bigint(20) NOT NULL auto_increment,
    "name" varchar(255) default NULL,
    "levelOfUgliness" int(11) default NULL,
    PRIMARY KEY  ("id")
) AUTO_INCREMENT=2;

Going back to my question:

I expect @ManyToOne person to be inherited by UglyProblem class.

I expect that because all other mapped fields are inherited and I do not see any reason to make this exception for ManyToOne relationships.

---

Yeah, I saw that. In fact, I used Read-Only solution for my case. But my question was "Why..." :). I know that there is an explanation given by a member of hibernate team. I was not able to find it and that is why I asked.

I want to find out the motivation of this design decision.

(if you interested how I have faced this problem: I inherited a project built using hibernate 3. It was Jboss 4.0.something + hibernate was already there (you'd download it all together). I was moving this project to Jboss 4.2.2 and I found out that there are inherited mappings of "@OneToMany mappedBy" and it worked fine on old setup...)

Answer 1

In my case I wanted to use the SINGLE_TABLE inheritance type, so using @MappedSuperclass wasn't an option.

What works, although not very clean, is to add the Hibernate proprietary @Where clause to the @OneToMany association to force the type in queries:

@OneToMany(mappedBy="person")
@Where(clause="DTYPE='UP'")
private List< UglyProblem > problems;