only calculate the klass once

activerecord/lib/active_record/associations/association_scope.rb

@@ -89,10 +89,11 @@ def add_constraints(scope)
             scope = scope.joins(join(foreign_table, constraint))
           end
 
+          klass = i == 0 ? self.klass : reflection.klass
+
           # Exclude the scope of the association itself, because that
           # was already merged in the #scope method.
           scope_chain[i].each do |scope_chain_item|
-            klass = i == 0 ? self.klass : reflection.klass
             item  = eval_scope(klass, scope_chain_item)
 
             if scope_chain_item == self.reflection.scope