Cat Gor = new Cat(){ IsAlive = true};
   MongoGorCollection.Save(Gor);
   //Saving Gor as a main level document

   Cage CatCage = new Cage(){ Animals.AddRange( new Cat[] { Gor }  )};
   MongoCageCollection.Save(CatCage);
   //Adding Gor to Animals list of Cage as a sub-document

   var RetrieveGor = MongoGorCollection.FirstOrDefault();   
   RetrieveGor.IsAlive = false;
   MongoGorCollection.Save(RetrieveGor);
   //Well, we have changed IsAlive of Gor to false and saved back in DB.
   // So if I retrieve the only cat in the Cage then it must be dead.

   
   bool IsAliveShouldBeFalse = MongoCageCollection.FirstOrDefault().Animals.FirstOrDefault().IsAlive;

   //IsAliveShouldBeFalse is not false! It's true.