Raise an error for associations which try to go :through a polymorphic association [#6212 state:resolved]

activerecord/lib/active_record/associations.rb

@@ -20,12 +20,18 @@ module ActiveRecord
     end
   end
 
-  class HasManyThroughAssociationPolymorphicError < ActiveRecordError #:nodoc:
+  class HasManyThroughAssociationPolymorphicSourceError < ActiveRecordError #:nodoc:
     def initialize(owner_class_name, reflection, source_reflection)
       super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' on the polymorphic object '#{source_reflection.class_name}##{source_reflection.name}'.")
     end
   end
 
+  class HasManyThroughAssociationPolymorphicThroughError < ActiveRecordError #:nodoc:
+    def initialize(owner_class_name, reflection)
+      super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' which goes through the polymorphic association '#{owner_class_name}##{reflection.through_reflection.name}'.")
+    end
+  end
+
   class HasManyThroughAssociationPointlessSourceTypeError < ActiveRecordError #:nodoc:
     def initialize(owner_class_name, reflection, source_reflection)
       super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' with a :source_type option if the '#{reflection.through_reflection.class_name}##{source_reflection.name}' is not polymorphic.  Try removing :source_type on your association.")

activerecord/lib/active_record/reflection.rb

@@ -375,6 +375,10 @@ module ActiveRecord
           raise HasManyThroughAssociationNotFoundError.new(active_record.name, self)
         end
 
+        if through_reflection.options[:polymorphic]
+          raise HasManyThroughAssociationPolymorphicThroughError.new(active_record.name, self)
+        end
+
         if source_reflection.nil?
           raise HasManyThroughSourceAssociationNotFoundError.new(self)
         end
@@ -384,7 +388,7 @@ module ActiveRecord
         end
 
         if source_reflection.options[:polymorphic] && options[:source_type].nil?
-          raise HasManyThroughAssociationPolymorphicError.new(active_record.name, self, source_reflection)
+          raise HasManyThroughAssociationPolymorphicSourceError.new(active_record.name, self, source_reflection)
         end
 
         unless [:belongs_to, :has_many, :has_one].include?(source_reflection.macro) && source_reflection.options[:through].nil?

activerecord/test/cases/associations/join_model_test.rb

@@ -340,11 +340,16 @@ class AssociationsJoinModelTest < ActiveRecord::TestCase
   end
 
   def test_has_many_polymorphic
-    assert_raise ActiveRecord::HasManyThroughAssociationPolymorphicError do
+    assert_raise ActiveRecord::HasManyThroughAssociationPolymorphicSourceError do
-      assert_equal posts(:welcome, :thinking), tags(:general).taggables
+      tags(:general).taggables
     end
+
+    assert_raise ActiveRecord::HasManyThroughAssociationPolymorphicThroughError do
+      taggings(:welcome_general).things
+    end
+
     assert_raise ActiveRecord::EagerLoadPolymorphicError do
-      assert_equal posts(:welcome, :thinking), tags(:general).taggings.find(:all, :include => :taggable, :conditions => 'bogus_table.column = 1')
+      tags(:general).taggings.find(:all, :include => :taggable, :conditions => 'bogus_table.column = 1')
     end
   end
 

activerecord/test/models/tagging.rb

@@ -7,4 +7,5 @@ class Tagging < ActiveRecord::Base
   belongs_to :super_tag,   :class_name => 'Tag', :foreign_key => 'super_tag_id'
   belongs_to :invalid_tag, :class_name => 'Tag', :foreign_key => 'tag_id'
   belongs_to :taggable, :polymorphic => true, :counter_cache => true
-end
+  has_many :things, :through => :taggable
+end