First / FirstOrDefault doesn't work as I would expect

I am using nhibernate and I wrote a linq query and it does not return what I expected.

public ParentA()
{
    public virtual Id {get; set;}
    public virtual Name {get; set;}
    public virtual IList<ChildA> ChidrenA {get; set;}

   public ParentA()
   {
       ChidrenA = new List<ChildA>();      
   }
}

public ChildA()
{
    public virtual Id {get; set;}
    public virtual Name {get; set;}
    public virtual IList<ChildB> ChidrenB {get; set;}
    public virtual ParentA ParenteA {get; set;}
   public ChildA()
   {
       ChidrenB = new List<ChildB>();      
   }
}

public ChildB()
{
    public virtual Id {get; set;}
    public virtual Name {get; set;}
    public virtual ChildA {get; set;}
}

      

        
        
        
      

    

The above code is my domains. Free nhibernate would be very simple and nothing fancy was happening, so I didn't include it.

I have a request

base.unitOfWork.Session.Query<ParentA>()
  .Where(x => x.Id == parentAId)
  .FetchMany(x => x.ChildrenA)
  .ThenFetchMany(x => x.ChildrenB)
  .FirstOrDefault();

      

        
        
        
      

    

What i expected to see

It will find 1 or 0 parent entries. If it finds one entry, it will download all ChildrenA files and then all children.

What's happening

It finds 1 or 0 parent records. Then it only accepts 1 or 0 records for ChildrenA and ChildrenB.

Why only the first found record for ChildrenA and ChildrenB?

If I change FirstToDefault()

to .toList()

, I get everything I expect, but I find it pointless as there should only be one entry with this parent entry.