Don't include inheritance column in the through_scope_attributes

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -93,7 +93,7 @@ module ActiveRecord
         end
 
         def through_scope_attributes
-          scope.where_values_hash(through_association.reflection.name.to_s).except!(through_association.reflection.foreign_key)
+          scope.where_values_hash(through_association.reflection.name.to_s).except!(through_association.reflection.foreign_key, through_association.reflection.klass.inheritance_column)
         end
 
         def save_through_record(record)

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -1139,4 +1139,12 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     assert_equal 2, post.lazy_readers_unscope_skimmers.to_a.size
     assert_equal 2, post.lazy_people_unscope_skimmers.to_a.size
   end
+  
+  def test_has_many_through_add_with_sti_middle_relation
+    club = SuperClub.create!(name: 'Fight Club')
+    member = Member.create!(name: 'Tyler Durden')
+
+    club.members << member
+    assert_equal 1, SuperMembership.where(member_id: member.id, club_id: club.id).count
+  end
 end

activerecord/test/models/club.rb

@@ -5,7 +5,7 @@ class Club < ActiveRecord::Base
   has_one :sponsor
   has_one :sponsored_member, :through => :sponsor, :source => :sponsorable, :source_type => "Member"
   belongs_to :category
-
+  
   has_many :favourites, -> { where(memberships: { favourite: true }) }, through: :memberships, source: :member
 
   private
@@ -14,3 +14,10 @@ class Club < ActiveRecord::Base
     "I'm sorry sir, this is a *private* club, not a *pirate* club"
   end
 end
+
+class SuperClub < ActiveRecord::Base
+  self.table_name = "clubs"
+  
+  has_many :memberships, :class_name => "SuperMembership", :foreign_key => "club_id"
+  has_many :members, :through => :memberships
+end