Chapter 21. Example: Parent/Child
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## Chapter 21. Example: Parent/Child

One of the very first things that new users try to do with Hibernate is to model a parent / child type relationship. There are two different approaches to this. For various reasons the most convenient approach, especially for new users, is to model both `Parent` and `Child` as entity classes with a `<one-to-many>` association from `Parent` to `Child`. (The alternative approach is to declare the `Child` as a `<composite-element>`.) Now, it turns out that default semantics of a one to many association (in Hibernate) are much less close to the usual semantics of a parent / child relationship than those of a composite element mapping. We will explain how to use a bidirectional one to many association with cascades to model a parent / child relationship efficiently and elegantly. It's not at all difficult!

## 21.1. A note about collections

Hibernate collections are considered to be a logical part of their owning entity; never of the contained entities. This is a crucial distinction! It has the following consequences:

- When we remove / add an object from / to a collection, the version number of the collection owner is incremented.

- If an object that was removed from a collection is an instance of a value type (eg, a composite element), that object will cease to be persistent and its state will be completely removed from the database. Likewise, adding a value type instance to the collection will cause its state to be immediately persistent.

- On the other hand, if an entity is removed from a collection (a one-to-many or many-to-many association), it will not be deleted, by default. This behaviour is completely consistent - a change to the internal state of another entity should not cause the associated entity to vanish! Likewise, adding an entity to a collection does not cause that entity to become persistent, by default.

Instead, the default behaviour is that adding an entity to a collection merely creates a link between the two entities, while removing it removes the link. This is very appropriate for all sorts of cases. Where it is not appropriate at all is the case of a parent / child relationship, where the life of the child is bound to the lifecycle of the parent.

## 21.2. Bidirectional one-to-many

Suppose we start with a simple `<one-to-many>` association from `Parent` to `Child`.

    <set name="children">
        <key column="parent_id"/>
        <one-to-many class="Child"/>
    </set>

If we were to execute the following code

    Parent p = .....;
    Child c = new Child();
    p.getChildren().add(c);
    session.save(c);
    session.flush();

Hibernate would issue two SQL statements:

- an `INSERT` to create the record for `c`

- an `UPDATE` to create the link from `p` to `c`

This is not only inefficient, but also violates any `NOT NULL` constraint on the `parent_id` column. We can fix the nullability constraint violation by specifying `not-null="true"` in the collection mapping:

    <set name="children">
        <key column="parent_id" not-null="true"/>
        <one-to-many class="Child"/>
    </set>

However, this is not the recommended solution.

The underlying cause of this behaviour is that the link (the foreign key `parent_id`) from `p` to `c` is not considered part of the state of the `Child` object and is therefore not created in the `INSERT`. So the solution is to make the link part of the `Child` mapping.

    <many-to-one name="parent" column="parent_id" not-null="true"/>

(We also need to add the `parent` property to the `Child` class.)

Now that the `Child` entity is managing the state of the link, we tell the collection not to update the link. We use the `inverse` attribute.

    <set name="children" inverse="true">
        <key column="parent_id"/>
        <one-to-many class="Child"/>
    </set>

The following code would be used to add a new `Child`

    Parent p = (Parent) session.load(Parent.class, pid);
    Child c = new Child();
    c.setParent(p);
    p.getChildren().add(c);
    session.save(c);
    session.flush();

And now, only one SQL `INSERT` would be issued!

To tighten things up a bit, we could create an `addChild()` method of `Parent`.

    public void addChild(Child c) {
        c.setParent(this);
        children.add(c);
    }

Now, the code to add a `Child` looks like

    Parent p = (Parent) session.load(Parent.class, pid);
    Child c = new Child();
    p.addChild(c);
    session.save(c);
    session.flush();

## 21.3. Cascading lifecycle

The explicit call to `save()` is still annoying. We will address this by using cascades.

    <set name="children" inverse="true" cascade="all">
        <key column="parent_id"/>
        <one-to-many class="Child"/>
    </set>

This simplifies the code above to

    Parent p = (Parent) session.load(Parent.class, pid);
    Child c = new Child();
    p.addChild(c);
    session.flush();

Similarly, we don't need to iterate over the children when saving or deleting a `Parent`. The following removes `p` and all its children from the database.

    Parent p = (Parent) session.load(Parent.class, pid);
    session.delete(p);
    session.flush();

However, this code

    Parent p = (Parent) session.load(Parent.class, pid);
    Child c = (Child) p.getChildren().iterator().next();
    p.getChildren().remove(c);
    c.setParent(null);
    session.flush();

will not remove `c` from the database; it will ony remove the link to `p` (and cause a `NOT NULL` constraint violation, in this case). You need to explicitly `delete()` the `Child`.

    Parent p = (Parent) session.load(Parent.class, pid);
    Child c = (Child) p.getChildren().iterator().next();
    p.getChildren().remove(c);
    session.delete(c);
    session.flush();

Now, in our case, a `Child` can't really exist without its parent. So if we remove a `Child` from the collection, we really do want it to be deleted. For this, we must use `cascade="all-delete-orphan"`.

    <set name="children" inverse="true" cascade="all-delete-orphan">
        <key column="parent_id"/>
        <one-to-many class="Child"/>
    </set>

Note: even though the collection mapping specifies `inverse="true"`, cascades are still processed by iterating the collection elements. So if you require that an object be saved, deleted or updated by cascade, you must add it to the collection. It is not enough to simply call `setParent()`.

## 21.4. Cascades and `unsaved-value`

Suppose we loaded up a `Parent` in one `Session`, made some changes in a UI action and wish to persist these changes in a new session by calling `update()`. The `Parent` will contain a collection of childen and, since cascading update is enabled, Hibernate needs to know which children are newly instantiated and which represent existing rows in the database. Lets assume that both `Parent` and `Child` have genenerated identifier properties of type `Long`. Hibernate will use the identifier and version/timestamp property value to determine which of the children are new. (See Section 10.7, “Automatic state detection”.) In Hibernate3, it is no longer necessary to specify an `unsaved-value` explicitly.

The following code will update `parent` and `child` and insert `newChild`.

    //parent and child were both loaded in a previous session
    parent.addChild(child);
    Child newChild = new Child();
    parent.addChild(newChild);
    session.update(parent);
    session.flush();

Well, that's all very well for the case of a generated identifier, but what about assigned identifiers and composite identifiers? This is more difficult, since Hibernate can't use the identifier property to distinguish between a newly instantiated object (with an identifier assigned by the user) and an object loaded in a previous session. In this case, Hibernate will either use the timestamp or version property, or will actually query the second-level cache or, worst case, the database, to see if the row exists.

## 21.5. Conclusion

There is quite a bit to digest here and it might look confusing first time around. However, in practice, it all works out very nicely. Most Hibernate applications use the parent / child pattern in many places.

We mentioned an alternative in the first paragraph. None of the above issues exist in the case of `<composite-element>` mappings, which have exactly the semantics of a parent / child relationship. Unfortunately, there are two big limitations to composite element classes: composite elements may not own collections, and they should not be the child of any entity other than the unique parent.

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