In the previous post we saw how to map an entity with a composite-id.
Well but now if you have mapped an entity in this way and you need to create a collection of this entity or maybe you have a related child class that uses this primary keys … you are in trouble! 
The way to solve the problem is very easy. First of a short example to understand what I’m talking about:
1: class Foo {
2: public string name { get; set; }
3: public string lastname { get; set; }
4: public IList<childfoo> listofchild { get; set; }
5: }
6: class childfoo {
7: public string name { get; set; }
8: public string lastname { get; set; }
9: public Foo father { get; set; }
10: }
Now the first class will be mapped in this way if we assume that “name and lastname are the primary keysâ€. (Also remember to override Equals() and GetHashCode()!!).
1: <class name="Foo" table="[Foo]" schema="xxx">
2: <composite-id>
3: <key-property name="name">
4: <column name="[name]" sql-type="varchar" length="50" not-null="true"/>
5: </key-property>
6: <key-property name="lastname">
7: <column name="[lastname]" sql-type="varchar" length="50" not-null="true"/>
8: </key-property>
9: </composite-id>
1: <bag name="listofchild" table="[childfoo]" inverse="true"
2: cascade="all-delete-orphan" generic="true" lazy="true">
3: <key>
4: <column name="[name]" sql-type="varchar"/>
5: <column name="[lastname]" sql-type="varchar"/>
6: </key>
7: <one-to-many class="Foo" not-found="ignore"/>
8: </bag>
Now we can go back to the child mapping file and reference the many-to-one foreign mapping in this simple way:
1: <many-to-one name="Foo" lazy="proxy" not-null="false">
2: <column name="[name]" sql-type="varchar"/>
3: <column name="[lastname]" sql-type="varchar"/>
4: </many-to-one>
So now my repository will be able to give me:
And in that moment my repository will go back and execute the sub-select.