References column name is pk

Hi,

I have the following classes:

class A
{ int pk; string name; B classB } class B
{ A classA; string name; }

I need make a reference between A and B but an error happens in insert, because i use the PK in Id and References

Id(x=>x.PK, "PK");
References(x => x.B, "PK");

what can i do?