class A
{
   int Id {get;set;}
   IEnumerable<B> BList {get;set;}
}

class B 
{
   int Id {get;set;}
   bool Flag {get;set;}
   A Parent {get;set;}
}

class AMap : ClassMap<A>
    {
        internal AMap()
        {
            Table("A");
            Id(x => x.Id);
            HasMany(e => e.BList)
                .Cascade.All()
                .Inverse()
                .Table("B")
                .KeyColumn("ParentId").Not.LazyLoad();
        }

  IEnumerable<A> aList = session.QueryOver<A>(() => a)
                      .JoinQueryOver<B>(e => e.BList).Where(e => e.Parent.Id == a.Id && e.Count > 0)
                      .TransformUsing(Transformers.DistinctRootEntity)
                      .List<A>();

  IEnumerable<A> aList = session.QueryOver<A>(() => a)
                      .JoinQueryOver<B>(e => e.BList).Where(e => e.Parent.Id == a.Id && e.Flag ==true)
                      .TransformUsing(Transformers.DistinctRootEntity)
                      .List<A>();