Mapping a date to a date range

Hi,
Wonder if this is possible and if can help;

public class ClassA
{
  long Id 
  DateTime FirstDay 
  DateTime LastDay 
  ...
}

public class ClassB
{
  DateTime ADate
  ClassA A
  ...
}

class ClassAMap
{
    CompositeId()
      .KeyProperty(x => x.Id, "ID")
      .KeyProperty(x => x.FirstDay, "FIRST_DAY")
      .KeyProperty(x => x.LastDay, "LAST_DAY"); 
    ...
}

The key of ClassB includes ADate and A (the reference to ClassA) but I need the resulting mapping to be such that ClassB.ADate between ClassA.FirstDay and ClassB.LastDay both in code and resulting SQL like;

from TableB b
inner join TableA a
  on b.TableA_ID = a.ID and b.ADATE between a.FIRST_DAY and a.LAST_DAY

Hope you can help as i am running out of ideas!

Many Thanks