主页
产品
Red Hat JBoss Enterprise Application Platform
5
Hibernate Core Reference Guide
21.2. Bidirectional one-to-many

此内容没有您所选择的语言版本。

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>

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

Copy to Clipboard

Toggle word wrap

If we were to execute the following code:

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

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

Copy to Clipboard

Toggle word wrap

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. You 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>

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

Copy to Clipboard

Toggle word wrap

However, this is not the recommended solution.

The underlying cause of this behavior 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. The solution is to make the link part of the Child mapping.

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

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

Copy to Clipboard

Toggle word wrap

You 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 to do this:

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

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

Copy to Clipboard

Toggle word wrap

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();

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

Copy to Clipboard

Toggle word wrap

Only one SQL INSERT would now be issued.

You could also create an addChild() method of Parent.

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

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

Copy to Clipboard

Toggle word wrap

The code to add a Child looks like this:

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

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

Copy to Clipboard

Toggle word wrap

返回顶部

此内容没有您所选择的语言版本。

21.2. Bidirectional one-to-many

学习

尝试、购买和销售

社区

关于红帽文档

让开源更具包容性

關於紅帽

Theme

Red Hat legal and privacy links

Red Hat legal and privacy links