Friday, October 5, 2007

Understanding "inverse" mapping attribute

Generality

This page intends to give an internal view and understanding of inverse="true". Please, please, please read the Hibernate reference guide and especially:

  • Mapping a collection
  • Bidirectional Association
  • Parent Child Relationships

and the FAQs (the official ones and the one from the Wiki) before reading this.

Inverse defines which side is responsible of the association maintenance. The side having inverse="false" (default value) has this responsibility (and will create the appropriate SQL query - insert, update or delete). Changes made to the association on the side of the inverse="true" are not persisted in DB.

Inverse attribute is not related in any way to the navigation through relationship. It is related to the way hibernate generate SQL queries to update association data. Association data are:

  • a column in the one-to-many association
  • a row in the association table in a many-to-many association

Monodirectional association is managed by the only side available through navigation. When association is bidirectional, choosing the manager side allow better SQL optimization, this is the recommended behaviour.

one-to-many sample

Let's have a look at a simple one-to-many sample. Setting inverse="true" is recommanded and allow SQL optimization.

Note that <many-to-one> is always inverse="false" (the attribute does not exist).

<class name="net.sf.test.Parent" table="parent">
<id name="id" column="id" type="long" unsaved-value="null">
<generator class="sequence">
<param name="sequence">SEQ_DEFAULT</param>
</generator>
</id>
<set name="children" lazy="true" inverse="true">
<key column="parent_id"/>
<one-to-many class="net.sf.test.Child"/>
</set>
</class>

<class name="net.sf.test.Child" table="child">
<id name="id" column="id" type="long" unsaved-value="null">
<generator class="sequence">
<param name="sequence">SEQ_DEFAULT</param>
</generator>
</id>
<many-to-one name="parent" column="parent_id" not-null="true"/>
</class>

The inverse="true" is set to the one side.

Proper code

Parent p = new Parent();
Child c = new Child();
p.setChildren(new HashSet());
p.getChildren().add(c);
c.setParent(p);

session.save(p);
session.save(c);
session.flush();

Will do the following SQL queries

Hibernate: select SEQ_DEFAULT.nextval from dual
Hibernate: select SEQ_DEFAULT.nextval from dual
Hibernate: insert into parent (id) values (?)
Hibernate: insert into child (parent_id, id) values (?, ?)

Hibernate insert parent then insert child. Note that my DB has a not null FK constraint on Child(parent_id), inserts work fine because I set <many-to-one not-null="true"

Note that I explicitly save parent and child objets. A better way is to use the cascade="save-update" element. I didn't do it to keep this explanation easier to understand and avoid concepts mismatch.

inverse="true" sample

Insert

Parent p = new Parent();
Child c = new Child();
p.setChildren(new HashSet());
p.getChildren().add(c);
c.setParent(p);

session.save(p);
session.flush(); //flush to DB
System.out.println("Parent saved");

session.save(c);
System.out.println("Child saved");
session.flush(); //flush to DB

Will do the following SQL queries

Hibernate: select SEQ_DEFAULT.nextval from dual
Hibernate: insert into parent (id) values (?)
Parent saved
Hibernate: select SEQ_DEFAULT.nextval from dual
Hibernate: insert into child (parent_id, id) values (?, ?)
Child saved

As you can see the relationship (incarnated by the parent_id column) is set during the child save : this is of the child responsibility. When saving parent, nothing is done on the relationship.

Update

Let's have a look at a relationship update

Parent p = (Parent) session.load(Parent.class, parentId);
Parent p2 = (Parent) session.load(Parent.class, parentId2);

c = (Child) session.find(
"from Child as child where child.parent = ?",
p, Hibernate.entity(Parent.class)).get(0);

// change parent of child c from p to p2
p.getChildren().remove(c);
p2.getChildren().add(c);
c.setParent(p2);

Will do the following SQL queries

Hibernate: select parent0_.id as id from parent parent0_ where parent0_.id=? //get parent 1
Hibernate: select parent0_.id as id from parent parent0_ where parent0_.id=? //get parent 2
Hibernate: select child0_.id as id, child0_.parent_id as parent_id from child child0_ where (child0_.parent_id=? ) //get children of parent 1

Hibernate: select child0_.id as id__, child0_.id as id, child0_.parent_id as parent_id from child child0_ where child0_.parent_id=?
Hibernate: select child0_.id as id__, child0_.id as id, child0_.parent_id as parent_id from child child0_ where child0_.parent_id=?
//load childrens of Parent 1 and 2 (can't avoid this with set, see FAQ)

Hibernate: update child set parent_id=? where id=?

After a proper Java setting of the Parent child relationship (both side), Hibernate, set parent_id column to the proper value. As you can see, only 1 update is executed.

Now, we'll see inverse="true" in action ;-)

Parent p = (Parent) session.load(Parent.class, parentId);
Parent p2 = (Parent) session.load(Parent.class, parentId2);

c = (Child) session.find(
"from Child as child where child.parent = ?",
p, Hibernate.entity(Parent.class)).get(0);

c.setParent(p2);

Will do the following SQL queries

Hibernate: select parent0_.id as id from parent parent0_ where parent0_.id=? //get parent 1
Hibernate: select parent0_.id as id from parent parent0_ where parent0_.id=? //get parent 2
Hibernate: select child0_.id as id, child0_.parent_id as parent_id from child child0_ where (child0_.parent_id=? ) //get children

Hibernate: update child set parent_id=? where id=?

The relationship is updated because I change it on the child side. Note that the object tree is not consistent with the Database (children collections are not up to date). This is not recommanded.

On the contrary,

Parent p = (Parent) session.load(Parent.class, parentId);
Parent p2 = (Parent) session.load(Parent.class, parentId2);

c = (Child) session.find(
"from Child as child where child.parent = ?",
p, Hibernate.entity(Parent.class)).get(0);

p.getChildren().remove(c);
p2.getChildren().add(p);

Will do the following SQL queries

Hibernate: select parent0_.id as id from parent parent0_ where parent0_.id=? //get parent 1
Hibernate: select parent0_.id as id from parent parent0_ where parent0_.id=? //get parent 2
Hibernate: select child0_.id as id, child0_.parent_id as parent_id from child child0_ where (child0_.parent_id=? ) //get children

Hibernate: select child0_.id as id__, child0_.id as id, child0_.parent_id as parent_id from child child0_ where child0_.parent_id=?
Hibernate: select child0_.id as id__, child0_.id as id, child0_.parent_id as parent_id from child child0_ where child0_.parent_id=?
//load childrens of Parent 1 and 2 (can't avoid this see FAQ)

Relationship update is not executed because update is only done on the parent side.

inverse="false"

inverse="false" (the default value) is not optimized for bidirectional relationships.

<class name="net.sf.test.Parent" table="parent">
<id name="id" column="id" type="long" unsaved-value="null">
<generator class="sequence">
<param name="sequence">SEQ_DEFAULT</param>
</generator>
</id>
<set name="children" lazy="true" inverse="false">
<key column="parent_id"/>
<one-to-many class="net.sf.test.Child"/>
</set>
</class>

<class name="net.sf.test.Child" table="child">
<id name="id" column="id" type="long" unsaved-value="null">
<generator class="sequence">
<param name="sequence">SEQ_DEFAULT</param>
</generator>
</id>
<many-to-one name="parent" column="parent_id" not-null="true"/>
</class>

The inverse="false" is set to the one side.

insert

Parent p = new Parent();
Child c = new Child();
p.setChildren(new HashSet());
p.getChildren().add(c);
c.setParent(p);

session.save(p);
session.save(c);
session.flush();

Will do the following SQL queries

Hibernate: select SEQ_DEFAULT.nextval from dual
Hibernate: select SEQ_DEFAULT.nextval from dual
Hibernate: insert into parent (id) values (?)
Hibernate: insert into child (parent_id, id) values (?, ?)
Hibernate: update child set parent_id=? where id=?

Parent is responsible of the relationship. Hibernate insert parent, insert child then update the relationship (as a request to the parent). Two SQL orders are executed (one insert and one update) instead of one.

Note that I cannot do a flush between session.save(p) and session.save(c) because, parent, which is responsible of the relationship, needs a persistent child to play with.

update

Let's have a look at a relationship update

Parent p = (Parent) session.load(Parent.class, parentId);
Parent p2 = (Parent) session.load(Parent.class, parentId2);

c = (Child) session.find(
"from Child as child where child.parent = ?",
p, Hibernate.entity(Parent.class)).get(0);

p.getChildren().remove(c);
p2.getChildren().add(c);
c.setParent(p2);

Will do the following SQL queries

Hibernate: select parent0_.id as id from parent parent0_ where parent0_.id=?    //get parent 1
Hibernate: select parent0_.id as id from parent parent0_ where parent0_.id=? //get parent 2
Hibernate: select child0_.id as id, child0_.parent_id as parent_id from child child0_ where (child0_.parent_id=? )
//get first child for parent 1

Hibernate: select child0_.id as id__, child0_.id as id, child0_.parent_id as parent_id from child child0_ where child0_.parent_id=?
Hibernate: select child0_.id as id__, child0_.id as id, child0_.parent_id as parent_id from child child0_ where child0_.parent_id=?
//load childrens of Parent 1 and 2 (can't avoid this see FAQ)

Hibernate: update child set parent_id=? where id=? // child.setParent
Hibernate: update child set parent_id=null where parent_id=? //remove
Hibernate: update child set parent_id=? where id=? // add

As you can see, having set inverse="true" allow the relationship to be managed by the parent side AND the child side. Several updates to the association data are done. This is inefficient considering the inverse="true" equivalent.

Parent p = (Parent) session.load(Parent.class, parentId);
Parent p2 = (Parent) session.load(Parent.class, parentId2);

c = (Child) session.find(
"from Child as child where child.parent = ?",
p, Hibernate.entity(Parent.class)).get(0);

p2.getChildren().add(c);

Will do the following SQL queries

Hibernate: select parent0_.id as id from parent parent0_ where parent0_.id=?    //get parent 1
Hibernate: select parent0_.id as id from parent parent0_ where parent0_.id=? //get parent 2
Hibernate: select child0_.id as id, child0_.parent_id as parent_id from child child0_ where (child0_.parent_id=? )
//get first child for parent 1

Hibernate: select child0_.id as id__, child0_.id as id, child0_.parent_id as parent_id from child child0_ where child0_.parent_id=?
Hibernate: select child0_.id as id__, child0_.id as id, child0_.parent_id as parent_id from child child0_ where child0_.parent_id=?
//load childrens of Parent 1 and 2 (can't avoid this see FAQ)

Hibernate: update child set parent_id=? where id=? // add

The relationship is properly set but the object model is in inconsistent state. This is not recommanded.

Conclusion

Using and understanding inverse="true" is essential to optimize your code. Prefer using inverse="true" on bidirectional association. After this tutorial it will be soooo easy ;-)

Source: simoes.org

2 Comments:

samsonych.com said...

Helpful link about "inverse attribute" of Hibernate - Hibernates bizarre interpretation of inverse

Anonymous said...

Nice article. One comment. I think the author may have meant inverse="false" in the first occurrence of the inverse attribute in the passage below:

"As you can see, having set inverse="true" allow the relationship to be managed by the parent side AND the child side. Several updates to the association data are done. This is inefficient considering the inverse="true" equivalent."