Multple Composite PrimaryKey reference as foreign key map in Fluent nHibernate

Question

This is a legacy table structure hence no Schema modification is possible.

Patient
-------
Code - PK
HospitalCode - PK - FK with Hospital.Code
HospitalGroup - PK - FK with Hospital.HospitalGroup
Other fields

Hospital
-------
Code - PK
HospitalGroup - PK - FK with HospitalGroup.Code
Other fields

Department
-------
Code - PK
HospitalCode - PK - FK with Hospital.Code
HospitalGroup - PK - FK with HospitalGroup.Code
Other fields

Diagnose
-------
PatientCode - PK - FK with Patient.Code
HospitalCode - PK - FK with Hospital.Code
HospitalGroup - PK - FK with Hospital.HospitalGroup
DepartmentCode - PK - FK with Department.Code
Other fields

The Mapping looks like the following

public class PatientMap : ClassMap<Patient>
{
    public PatientMap()
    {
        CompositeId()
           .KeyProperty(x => x.Id, "ID")
           .KeyReference(x => x.Hospital, "Hospital_id", "HospitalGroup_id")
           .KeyReference(x => x.HospitalGroup, "HospitalGroup_id");

        Map(x => x.Name);            
    }
}

public class HospitalMap : ClassMap<Hospital>
{
    public HospitalMap()
    {
        CompositeId()
          .KeyProperty(x => x.Id, "ID")              
          .KeyReference(x => x.HospitalGroup, "HospitalGroup_id");

        Map(x => x.Name);            
    }
}

public class HospitalGroupMap : ClassMap<HospitalGroup>
{
    public HospitalGroupMap()
    {
        Id(x => x.Id).Column("ID");            
        Map(x => x.Name);                        
    }
}    

public class DepartmentMap : ClassMap<Department>
{
    public DepartmentMap()
    {
       CompositeId()
           .KeyProperty(x => x.Id, "ID")
           .KeyReference(x => x.Hospital, "Hospital_id", "HospitalGroup_id")
           .KeyReference(x => x.HospitalGroup, "HospitalGroup_id");

        Map(x => x.Name);                      
    }
}

public class DiagnoseMap : ClassMap<Diagnose>
{
    public DiagnoseMap()
    {
        CompositeId()
            //.KeyReference(x => x.Hospital, "Hospital_id", "HospitalGroup_id")
            //.KeyReference(x => x.HospitalGroup, "HospitalGroup_id")
            .KeyReference(x => x.Department, "Department_id", "Hospital_id", "HospitalGroup_id")
            .KeyReference(x => x.Patient, "Patient_id", "Hospital_id", "HospitalGroup_id");
    }
}

How to make the fluent mapping such that I will be able to get

Patient.DiagnoseList[0].Department.Hospital.Code

Currently I am getting the following exception

"identifier mapping has wrong number of columns: Examples.FirstProject.Entities.Diagnose type: component[Department,Patient]"