class Bullet::Detector::Association

def impossible_objects

impossible_objects are used to avoid treating 1+1 query to N+1 query.
if find collection returns only one object, then the object is impossible object,
e.g. { Post => ["Post:1", "Post:2"] }
that the objects may not cause N+1 query.
impossible_objects keep the class to objects relationships 
def impossible_objects
  Thread.current.thread_variable_get(:bullet_impossible_objects)
end