@OneToMany and composite primary keys?

Question

I'm using Hibernate with annotations (in spring), and I have an object which has an ordered, many-to-one relationship which a child object which has a composite primary key, one component of which is a foreign key back to the id of the parent object.

The structure looks something like this:

+=============+                 +================+
| ParentObj   |                 | ObjectChild    |
+-------------+ 1          0..* +----------------+
| id (pk)     |-----------------| parentId       |
| ...         |                 | name           |
+=============+                 | pos            |
                                | ...            |
                                +================+

I've tried a variety of combinations of annotations, none of which seem to work. This is the closest I've been able to come up with:

@Entity
public class ParentObject {
    @Column(nullable=false, updatable=false)
    @Id @GeneratedValue(generator="...")
    private String id;

    @OneToMany(mappedBy="parent", fetch=FetchType.EAGER, cascade={CascadeType.ALL})
    @IndexColumn(name = "pos", base=0)
    private List<ObjectChild> attrs;

    ...
}

@Entity
public class ChildObject {
    @Embeddable
    public static class Pk implements Serializable {
        @Column(nullable=false, updatable=false)
        private String parentId;

        @Column(nullable=false, updatable=false)
        private String name;

        @Column(nullable=false, updatable=false)
        private int pos;

        @Override
        public String toString() {
            return new Formatter().format("%s.%s[%d]", parentId, name, pos).toString();
        }

        ...
    }

    @EmbeddedId
    private Pk pk;

    @ManyToOne
    @JoinColumn(name="parentId")
    private ParentObject parent;

    ...
}

I arrived at this after a long bout of experimentation in which most of my other attempts yielded entities which hibernate couldn't even load for various reasons.

UPDATE: Thanks all for the comments; I have made some progress. I've made a few tweaks and I think it's closer (I've updated the code above). Now, however, the issue is on insert. The parent object seems to save fine, but the child objects are not saving, and what I've been able to determine is that hibernate is not filling out the parentId part of the (composite) primary key of the child objects, so I'm getting a not-unique error:

org.hibernate.NonUniqueObjectException:
   a different object with the same identifier value was already associated 
   with the session: [org.kpruden.ObjectChild#null.attr1[0]]

I'm populating the name and pos attributes in my own code, but of course I don't know the parent ID, because it hasn't been saved yet. Any ideas on how to convince hibernate to fill this out?

Thanks!

Gree

Best of luck. A project I was on had to totally abandon composite keys in favor of synthetic keys because of poor support from Hibernate. – Immediacy

The exception might be caused by your using of List<>. You must use Set<> to avoid the key duplication by using hashCode and equals definition. Hibernate treats two "logically identical" elements in a list to be two elements, then caused the exception. – Unbecoming

Answer 1

After much experimentation and frustration, I eventually determined that I cannot do exactly what I want.

Ultimately, I went ahead and gave the child object its own synthetic key and let Hibernate manage it. It's a not ideal, since the key is almost as big as the rest of the data, but it works.

Gree

'@JoinColumns({ @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"), @JoinColumn(name="userlastname_fk", referencedColumnName="lastName") })' – Mackinnon

A good decision. Trying to do anything except basic stuff in Hibernate requires too much un-intuitive messing around. I usually do the same - give EVERYTHING a primary - it keeps things simpler for Hibernate to understand. – Craniometry

Answer 2

The Manning book Java Persistence with Hibernate has an example outlining how to do this in Section 7.2. Fortunately, even if you don't own the book, you can see a source code example of this by downloading the JPA version of the Caveat Emptor sample project (direct link here) and examining the classes Category and CategorizedItem in the auction.model package.

I'll also summarize the key annotations below. Do let me know if it's still a no-go.

ParentObject:

@Entity
public class ParentObject {
   @Id @GeneratedValue
   @Column(name = "parentId", nullable=false, updatable=false)
   private Long id;

   @OneToMany(mappedBy="parent", fetch=FetchType.EAGER)
   @IndexColumn(name = "pos", base=0)
   private List<ChildObject> attrs;

   public Long getId () { return id; }
   public List<ChildObject> getAttrs () { return attrs; }
}

ChildObject:

@Entity
public class ChildObject {
   @Embeddable
   public static class Pk implements Serializable {
       @Column(name = "parentId", nullable=false, updatable=false)
       private Long objectId;

       @Column(nullable=false, updatable=false)
       private String name;

       @Column(nullable=false, updatable=false)
       private int pos;
       ...
   }

   @EmbeddedId
   private Pk id;

   @ManyToOne
   @JoinColumn(name="parentId", insertable = false, updatable = false)
   @org.hibernate.annotations.ForeignKey(name = "FK_CHILD_OBJECT_PARENTID")
   private ParentObject parent;

   public Pk getId () { return id; }
   public ParentObject getParent () { return parent; }
}

Triumph

Sadly, that didn't help: I get the same error. The only changes I see from what I had are the addition of the @ForeignKey attribute, and the addition of the {insertable,updatable}=false on the parent attribute. Is there something else I'm not seeing? Thanks! – Gree

There are a few other changes. (1) Try changing the parentId data type to Long; (2) I believe that the getters are necessary; (3) pay very close attention to the column names for ParentObject#id, ParentObject#attrs, ChildObject.Pk#objectId, and ChildObject#parent; (4) The id field of ParentObject needs the @Id annotation (and @GeneratedValue if you're not manually supplying the id); (5) I renamed ChildObject#pk to ChildObject#id, although I don't think this change was necessary. If you've got all this, can you supply the reading code that's throwing with some surrounding context? – Triumph

Re (2) above: Getters and setters aren't necessary. Hibernate can construct proxies to do what needs doing. I find it best to consider this "magic". – Past

@John: Thanks for the clarification about getters/setters. @Kris: I also don't think that the parentId field actually needs to be Long; I'm just trying to minimize the number of differences between my working code and your code. – Triumph

You must use Set<> to avoid the key duplication by using hashCode and equals definition. Hibernate treats two "logically identical" elements in a list to be two elements – Unbecoming

Answer 3

You should incorporate the ParentObject reference just into ChildObject.Pk rather than map parent and parentId separately:

(getters, setters, Hibernate attributes not related to problem and member access keywords omitted)

class ChildObject { 
    @Embeddable
    static class Pk {
        @ManyToOne...
        @JoinColumn(name="parentId")
        ParentObject parent;

        @Column...
        String name...
        ...
    }

    @EmbeddedId
    Pk id;
}

In ParentObject you then just put @OneToMany(mappedBy="id.parent") and it works.

Leroy

In my case it caused StackOverflowException when hibernate tried to resolve @ManyToOne relationship ;c – Karalee

Worked great. The dot on mappedBy="id.parent" really makes the difference. – Offstage

Answer 4

Firstly, in the ParentObject, "fix" the mappedBy attribute that should be set to "parent". Also (but this is maybe a typo) add an @Id annotation:

@Entity
public class ParentObject {
    @Id
    @GeneratedValue
    private String id;

    @OneToMany(mappedBy="parent", fetch=FetchType.EAGER)
    @IndexColumn(name = "pos", base=0)
    private List<ObjectChild> attrs;

    // getters/setters
}

Then, in ObjectChild, add a name attribute to the objectId in the composite key:

@Entity
public class ObjectChild {
    @Embeddable
    public static class Pk implements Serializable {
        @Column(name = "parentId", nullable = false, updatable = false)
        private String objectId;

        @Column(nullable = false, updatable = false)
        private String name;

        @Column(nullable = false, updatable = false)
        private int pos;
    }

    @EmbeddedId
    private Pk pk;

    @ManyToOne
    @JoinColumn(name = "parentId", insertable = false, updatable = false)
    private ParentObject parent;

    // getters/setters

}

AND also add insertable = false, updatable = false to the @JoinColumn because we are repeating the parentId column in the mapping of this entity.

With these changes, persisting and reading the entities is working fine for me (tested with Derby).

Avenge

Just spent half an hour trying to solve this issue and my problem was that my legacy database had the foreign key in the table as "forecastId" and that's what I copied into my JPA annotation and for reasons unknown, Spring decided that the column must therefore be "forecast_id" in the database and it fell over. Must be an algorithm to automatically identify columns that was slightly wrong. I changed the @Column(name= to "forecastid" (all lower case) and Spring decided to behave. – Psalter

Answer 5

Found this question searching for the answer to it's problem, but it's answers didn't solve my problem, because I was looking for @OneToMany which isn't as good of a fit for the table structure I was going after. @ElementCollection is the right fit in my case. One of the gotchas of it I believe though is that it looks at the entire row of relations as being unique, not just the rows id.

@Entity
public class ParentObject {
@Column(nullable=false, updatable=false)
@Id @GeneratedValue(generator="...")
private String id;

@ElementCollection
@CollectionTable( name = "chidren", joinColumns = @JoinColumn( name = "parent_id" ) )
private List<ObjectChild> attrs;

...
}

@Embeddable
public static class ObjectChild implements Serializable {
    @Column(nullable=false, updatable=false)
    private String parentId;

    @Column(nullable=false, updatable=false)
    private String name;

    @Column(nullable=false, updatable=false)
    private int pos;

    @Override
    public String toString() {
        return new Formatter().format("%s.%s[%d]", parentId, name, pos).toString();
    }

    ... getters and setters REQUIRED (at least they were for me)
}

Abm

Answer 6

It seems that you got pretty close, and I am trying to do the same thing in my current system. I started with the surrogate key but would like to remove it in favor of a composite primary key consisting of the parent's PK and the index in the list.

I was able to get a one-to-one relationship that shares the PK from the master table by using a "foreign" generator:

@Entity
@GenericGenerator(
    name = "Parent",
    strategy = "foreign",
    parameters = { @Parameter(name = "property", value = "parent") }
)
public class ChildObject implements Serializable {
    @Id
    @GeneratedValue(generator = "Parent")
    @Column(name = "parent_id")
    private int parentId;

    @OneToOne(mappedBy = "childObject")
    private ParentObject parentObject;
    ...
}

I wonder if you could add the @GenericGenerator and @GeneratedValue to solve the problem of Hibernate not assigning the parent's newly acquired PK during insertion.

Attractant

Answer 7

After saving the Parent object, you have to explicitly set the parentId in the Child objects for the inserts on the Child objects to work.

Agenda

Answer 8

After spending three days on this, I think I have found a solution, but to be honest, I don't like it and it can definitely be improved. However, it works and solves our problem.

Here is your entity constructor, but you could also do it in the setter method. Also, I used a Collection object but it should be same or similar with List:

...
public ParentObject(Collection<ObjectChild> children) {
    Collection<ObjectChild> occ = new ArrayList<ObjectChild>();
    for(ObjectChild obj:children){
        obj.setParent(this);
        occ.add(obj);
    }
    this.attrs = occ;
}
...

Basically, as someone else suggested, we must first manually set all the children's parent id before saving the parent (along with all children)

Gamez

Answer 9

I was badly looking for an answer but couldn't find a working solution. Though I had the OneToMany in parent correctly and ManyToOne in child correctly, during parent's save, child's key was not getting assigned, the auto-generated value from parent.

My problem was fixed upon adding an annotation javax.persistence.MapsId above the @ManyToOne mapping in the child entity (Java class)

@MapsId("java_field_name_of_child's_composite_key_that_needs_the_value_from_parent")
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "PARENT_ID", nullable = false, insertable = false, updatable = false)
private Parent parent;

This is on top of what was answered by @Pascal Thivent (answered on Apr 10 '10 at 1:40)

Please refer to the example code snippet in his post, earlier in this thread.

Thanks, PJR.

Borlase