NHibernate many-to-one association refuses to load

Question

I have the following mapping, the many-to-one property 'Message' has a corresponding one-to-many association in the 'RootMessage' class.

<class name="IMessageReceipt" lazy="false" table="MessageReceipts" abstract="true">

        <id name="Id">
            <generator class="guid.comb"></generator>
        </id>

        <discriminator column="Discriminator"/>
        <property name="Address" />
        <property name="Status" />
        <property name="MarkedAsDeleted" />

        <many-to-one name="Message" column="MessageId" class="RootMessage" 
                not-found="ignore"/>

        <subclass name="MessageReceipt" lazy="false" discriminator-value="1">
        </subclass>

    </class>

The many-to-one association refuses to load when using the criteria api (all I get is NULL), here is an example of a query:

            List<IMessageReceipt> list;
        using (var tx = Session.BeginTransaction())
        {
            var criteria = Session.CreateCriteria(typeof (IMessageReceipt));
            criteria.Add(Restrictions.Eq("Address", address));
            criteria.Add(Restrictions.Eq("Status", status));
            criteria.SetFirstResult(0);
            criteria.SetMaxResults(quantity);
            list = criteria.List<IMessageReceipt>().ToList();
            tx.Commit();
        }
        return list;

Any ideas?

Answer 1

Ok so after almost a day of chagrin I have the solution. NHibernate doesn't automatically assume a bi-directional association between two entities even if you have mappings between both. You need to imperatively declare the associations in your code before persisting. Thus:

message.Receipts = receipts;
foreach (var receipt in receipts)
{
   receipt.Message = message;
}
Session.Save(message);
tx.Commit();

Also inverse="true" should be applied to the side with the collection member:

<set name="Receipts" inverse="true" cascade="save-update">
<key column="MessageId"></key>
<one-to-many class="IMessageReceipt"/>
</set>