How do I use Fluent NHibernate to configure the following joins?

This is based on a legacy system.

I have the following tables:

CREATE TABLE a
  id int

CREATE TABLE b
  a_id int,
  c_id int
  relationshipid int -- must be IN (1, 2, 3)

CREATE TABLE c
  id int

I want the foll开发者_StackOverflow中文版owing domain models

public class A
{
  public int Id { get; set; }

  public C entityc { get ; set; }
}

public class C
{
  public int Id { get; set; }
}

Table b is set up so that for a particular defined relationshipid there is (well, should only be) one pair of ids. For other relationships, that one to one mapping through B doesn't hold true. Relationshipid can be one of a small number of values.

How do I get entity C into class A from the relationship where the relationshipid is 1 using fluent NHIbernate?

As a side question, is there a name for what I am trying to do here? The original approach was trying use a HasOne with a Join table and Filter the results, but obviously that failed miserably.

EDIT: Clarified RelationshipID and purpose.

---

I think the easiest way to map this would be to make your table b an entity and have references to both A and C within that entity and RelationshipId as the id. So your mappings would look something like this:

public class A
{
    public int Id { get; set; }

    public IList<B> bEntities { get; set; }
}

public class ClassAMap : ClassMap<A>
{
    public AMap()
    {
        Table("A");
        Id(x => x.Id);

        HasMany(x => x.bEntities)
            .KeyColumns.Add("a_id");
    }
}

public class B
{
    public virtual int RelationshipId { get; set; }
    public virtual A InstanceA { get; set; }
    public virtual C InstanceC { get; set; }
}

public class ClassBMap : ClassMap<B>
{
    public BMap()
    {
        Table("B");
        Id(x => x.RelationshipId , "relationshipid");

        References(x => x.InstanceA);
        References(x => x.InstanceC);
    }
}  

Edit:

If your wanting to filter these results for the collection of B entities in your A entity to only ones matching RelationshipId = 1 then you should take a look at this post:

Fluent NHibernate and filtering one-to-many relationship on query requiring multiple joins?

You could also do something like this in your class A:

public class A
{
    public int Id { get; set; }

    public IList<B> bEntities { get; set; }

    public C InstanceC
    {
       get { return bEntities.First<B>(x => x.RelationshipId == 1).InstanceC; }
    }
}