I use an ICD9 object in my Patient Diagnosis object.

public ICD9 diagnosisId
{
get { return _diagnosisId; }
set { _diagnosisId = value; }
}

Inside the ICD9 is a public Description field

I define the mapping file as this:
<many-to-one name="diagnosisId" column="diagnosisId" class="HospiceCare.Core.Domain.ICD9, HospiceCare.Core" />

My grid view has this templatefield and the SORTING ISN'T WORKING. BUT THE Description displays fine when reference, but the sorting bombs.

<asp:TemplateField HeaderText="Diagnosis" SortExpression ="diagnosisId.Description" >
<ItemTemplate ><a href="EditPatientDiagnosis.aspx?Action=Edit&PatientDiagnosisID=<%#Encryption.Encrypt(Eval("ID").ToString()) %>&PatientGUID=<%#Eval("PatientGUID").ToString()%>"><%# Eval("diagnosisId.Description")%></a>
</ItemTemplate></asp:TemplateField>

Please HELP. I have been searching for hours on how to do this. It's a simple concept that I can't get working.