NHibernate many-to-one with a composite-id entity

Question

How can I map a many-to-one relationship where the "one" entity has a composite-id? For example:

public class SingleEntity  
{  
  public int FirstId{get;set;}  
  public int SecondId{get;set;}  
  public string SomeData{get;set;}  
}  

The entity ManyEntity has (in addition to a primaryKey) columns for both SingleEntity 's FirstId and SecondId, so I want to be able to mape the relationship so that ManyEntity would look like that:

public class ManyEntity  
{  
public int Id{get;set;}  
public SingleEntity Single{get;set;}  
public string Name{get;set;}  
}  

How can I do it using NHibernate?

Answer 1

Fluent allows you to specify composite id mapping using CompositeId() method. However, as we can read in this very method documentation:

Note: Prefer using a surrogate key over a composite key whenever possible.

Composite ids usually require lot of effort for discussable/no gains. If you can , I would strongly advocate to use surrogate key. It's a lot easier.

Yet, if you're stuck with your DB schema, maybe those questions can help you:

• Fluent NHibernate and composite ID with single column name
• Fluent NHibernate — Saving Entity with Composite Key
• Fluent NHibernate Composite ID table problem

If you google those phrases (fluent nhibernate composite id), you'll quickly find out that awful lot of links somehow also contain word "problem" ...