Question

I’ve been struggling with this for a while see my previous question How to configure nHibernate for many-many mapping?

I have recreated my project from scratch in an attempt to solve it. I have two classes Broker and Instrument and I have a property unique to a Broker and an Instrument called MinIncrement that I need to store in my many-many mapping table in the database. I created a third class called BrokerInstrument to model this extra information. Here is the code for my classes:

public abstract class Entity
{
    public virtual int Version {get; private set;}
}

public class Broker : Entity
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual bool IsDefault { get; set; }
    public virtual bool IsActive { get; set; }
    public virtual ISet<BrokerInstrument> BrokerInstruments { get; set; }
}

public class Instrument : Entity
{
    public virtual int Id { get;  set; }
    public virtual string Name { get;  set; }
    public virtual string Symbol {get;  set;}
    public virtual ISet<BrokerInstrument> BrokerInstruments { get; set; }

}

public class BrokerInstrument : Entity
{
    public virtual Broker Broker { get; set; }
    public virtual Instrument Instrument { get; set; }
    public virtual decimal MinIncrement { get; set; }

    public override bool Equals(object obj)
    {
        if (obj == null)
            return false;

        var t = obj as BrokerInstrument;
        if (t == null)
            return false;

        if ((Instrument == t.Instrument) && (Broker == t.Broker))
            return true;

        return false;
    }

    public override int GetHashCode()
    {
        return (Broker.Name + "|" + Instrument.Symbol).GetHashCode();            
    }
}

My mapping files look like this:
Broker.hbm.xml

  <class name="MooDB.Domain.Broker,MooDB" table="brokers">
    <id name="Id" column="brokerId" type="Int32" unsaved-value="0">
      <generator class="native" />
    </id>
    <version name="Version" column="version" type="integer" unsaved-value="0" />
    <property name="Name" column="`name`" type="String" length="50" not-null="true" />
    <property name="IsActive" column="isActive" type="bool" not-null="true" />
    <property name="IsDefault" column="isDefault" type="bool" not-null="true" />
  </class>

Instrument.hbm.xml

  <class name="MooDB.Domain.Instrument,MooDB"  table="instruments">
    <id name="Id" column="instrumentId" type="Int32" unsaved-value="0">
      <generator class="native"></generator>
    </id>
    <version name="Version" column="version" type="integer" unsaved-value="0"/>
    <property name="Symbol" column="symbol" type="string" length="10" not-null="false"/>
    <property name="Name" column="`name`" type="string" length="30" not-null="false"/>
  </class>

And the mapping file I’m struggling with:
BrokerInstrument.hbm.xml

  <class name="MooDB.Domain.BrokerInstrument,MooDB"  table="brokerInstruments">
    <composite-id name="BrokerInstrumentIdentifier" class="BrokerInstrument" >
      <key-many-to-one class="Broker" column="brokerId" name="Id" />
      <key-many-to-one class="Instrument" column="instrumentId" name="Id" />
    </composite-id>
    <property name="MinIncrement" column="minIncrement" type="decimal" not-null="true" />
  </class>

I’m pretty sure the elements enclosed within composite-id tags are wrong. Do you know what should go in there? Also, does my class design look right to support this model?

UPDATE 1

In response to Cole’s reply. I have made the change suggested and I now get this error:

Test ‘Test.Test.CanAddBrokerInstrument’ failed: TestFixtureSetUp
failed in Test

TestFixture failed: NHibernate.PropertyNotFoundException : Could not
find a getter for property ‘BrokerInstrumentIdentifier’ in class
‘MooDB.Domain.BrokerInstrument’

Answer 1

this isn’t really an M:N relationship. it’s a M:1 & 1:N relationships between 3 entities. From simplicity of modeling and queries I would assign an Id to BrokerInstrument and remove the need for a composite key all together.

anytime you can avoid composite you should. this type of mapping is for legacy databases.