Two classes mapped to same table(one readonly) must be in correct order?

Question

Consider these two classes mapped to the same table. One is readonly via mutable="false".

<class name="Funder" table="funder">
    <id name="id">
      <generator class="identity" />
    </id>
    <property name="funder_name" />
    <property name="contact_name" />
    <property name="addr_line_1" />
    <property name="addr_line_2" />
    <property name="addr_line_3" />
    <property name="city" />
    <many-to-one name="state" column="state_id" foreign-key="FK_funder_state_id" fetch="join" />
    <property name="zip_code" length="10" />
    <property name="phone_number" length="30" />

    <property name="create_dt" update="false" not-null="true" />
    <many-to-one name="create_by" column="create_by" not-null="true" update="false" foreign-key="FK_funder_create_by" fetch="join" />
    <property name="last_update_dt" insert="false" />
    <many-to-one name="last_update_by" insert="false" foreign-key="FK_funder_last_update_by" fetch="join" />

  </class>

  <class name="FunderSimple" table="funder" schema-action="none" mutable="false">
    <id name="id">
      <generator class="identity" />
    </id>
    <property name="funder_name" />
    <property name="contact_name" />
    <property name="phone_number" />
  </class>

If I move the FunderSimple mapping before the Funder mapping my schema does not generate correctly. If I leave it as is above, it works.

Is this by design? It seems as though the schema-action="none" sticks to the table_name and later mappings to the same table will not generate the schema.

I'm doing it like this because I have another class named Contract which has a foreign key to the funder table. However, I don't need all the funder columns when referencing from the contract object.

<many-to-one name="funder_simple" column="funder_id"  foreign-key="FK_contract_funder_id" fetch="join" />

Funder does not inherit from FunderSimple.

Should I be using a different technique to fetch only a subset of columns from a foreign key table? Is many-to-one the only way to setup a foreign key?

using version 2.1.0.4000

Answer 1

For such situations, I use projections instead. I've never mapped two types to the same table (unless for inheritance reasons).

So, what I do in such a situation is:

create the FunderSimple class, and import it so that it is known by NHibernate:

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
    <import class="MyNamespace.FunderSimple" />
</hibernate-mapping>

Once you've done this, you can create a query on your 'Funder' type, with the ICriteria API, but, you could specify that you would like NHibernate to return instances of FunderSimple. By doing so, NHibernate is smart enough to generate a simplified SQL query, that only retrieves the columns that are necessary to populate instances of the FunderSimple class.

This is done like this:

ICriteria crit = session.CreateCriteria (typeof(Funder));
// add some expressions ...
crit.Add ( ... );

// Now, set the projection, and specify that FunderSimple should be returned
crit.SetProjection (Projections.ProjectionList()
                         .Add (Projections.Property ("Id"), "Id")
                         .Add (Projections.Property ("funder_name"), "funder_name")
                         .Add (Projections.Property ("phone_number"), "phone_number"));

crit.SetResultTransformer (Transformers.AliasToBean (typeof(FunderSimple)));

crit.List <FunderSimple>();