Hi!

I am new to NHibernate, so my question may be trivial. I have Parent-Child relationship (as one-to-many). I am trying to fetch all Parent objects with their Children. Currently, I have two parents in database, one with single child, the other with two children.

NHibernate version: 1.0.4.0

Mapping documents:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">
<class name="BusinessEntity.Parent, BusinessEntity" table="Parent">
<id name="Id" column="ParentId" type="Int32" unsaved-value="0">
<generator class="assigned" />
</id>
<property name="Name"/>
<property name="Gender"/>

<list name="Children" fetch="select">
<key column ="ParentId" />
<index column="ChildId" />
<one-to-many class="BusinessEntity.Child, BusinessEntity"/>
</list>
</class>
</hibernate-mapping>

<class name="BusinessEntity.Child, BusinessEntity" table="Child">
<id name="Id" column="ChildId" type="Int32" unsaved-value="0">
<generator class="assigned" />
</id>
<property name="Name"/>
<property name="Age"/>
<property name="ParentId"/>
</class>

Both Parent and child implement matching interfaces:


Code between sessionFactory.openSession() and session.close():


I fetch parents and first has collection of 4 children (with two NULL values) and the other 3 children (with two NULL values).

What seems to be the problem? I know I could use set (and ISet) instead of list (and IList), but I am trying to make it work with System.Collection namespace.

Thanks.

This is because you're using the ChildId as an index into the list. NHibernate follows list semantics, so if one parent has one child with ID=2, then that object will be loaded into parentObj.Children[2]. Since there are no children with ID's 0 or 1, parentObj.Children[0] and parentObj.Children[1] will be null.

If you really want to use a list, you should add another column to serve as the list index. After you add the property and map it, you won't need to worry about it again. You also may be able to use a Bag, which is implemented as an IList.