Many to Many Mapping on a single class

Hi Guys

I have come across a situation.

I have two tables Node and NodeValidParents as follows

Node

NodeId_PK
Name

NodeValidParents NodeId_FK (Foreign key from Node)
ValidParentNodeId_FK (Foreign key from Node)

A Node can have a list of ValidParents. As you can see from the tables, a ValidParent is also a node itself. So there is actually a many to many relationship ( A node can have many ValidParents and a ValidParent can belong to many nodes)

My Node class looks like this

public class Node
{
    public virtual int Id
    {
        get;
        set;
    }

    public virtual string Name
    {
        get;
        set;
    }

    public virtual IList<Node> ValidParents
    {
        get;
        protected set;
    }

    public Node()
    {
        ValidParents = new List<Node>();
    }

}

I don't have any class to represent ValidParent because I think there is no need for it, as ValidParent is itself a Node.

I am using autoMapping and I just can't figure out how to map this class to these two tables. I am implementing IAutoMappingOverride for overriding automappings for Node but I am not sure what to do in it.

public class NodeMap : IAutoMappingOverride

{
    public void Override(AutoMapping<Node> mapping)
    {
        //mapping.HasMany(x => x.ValidParents).Cascade.All().Table("NodeValidParents");
    }
}

Could anybody out there help me with this please?

Also I want to make sure that the combination of NodeId_FK and ValidParentNodeId_FK is always unique or Composite Key. I have seem people saying using composite keys is not the best practice and surrogate key should be used. I have no problem with using Surrogate key but I want to make sure that if a node already has a ValidParent , it can not be inserted again. How can i achieve this with fluent nhibernate mapping?

Just so that you know I have never used NHibernate with XML mapping. I have only used Fluent NHibernate.

Thanks
Nabeel