diff --git a/activerecord/lib/active_record/associations/has_many_through_association.rb b/activerecord/lib/active_record/associations/has_many_through_association.rb
index deb0f8c9f5..36fc381343 100644
--- a/activerecord/lib/active_record/associations/has_many_through_association.rb
+++ b/activerecord/lib/active_record/associations/has_many_through_association.rb
@@ -66,6 +66,11 @@ def build_through_record(record)
 
             through_record = through_association.build(*options_for_through_record)
             through_record.send("#{source_reflection.name}=", record)
+
+            if options[:source_type]
+              through_record.send("#{source_reflection.foreign_type}=", options[:source_type])
+            end
+
             through_record
           end
         end
diff --git a/activerecord/test/cases/reflection_test.rb b/activerecord/test/cases/reflection_test.rb
index 9c04a41e69..a130d9f7ca 100644
--- a/activerecord/test/cases/reflection_test.rb
+++ b/activerecord/test/cases/reflection_test.rb
@@ -23,6 +23,7 @@
 require 'models/department'
 require 'models/cake_designer'
 require 'models/drink_designer'
+require 'models/mocktail_designer'
 require 'models/recipe'
 
 class ReflectionTest < ActiveRecord::TestCase
@@ -278,6 +279,14 @@ def test_scope_chain_does_not_interfere_with_hmt_with_polymorphic_case
     assert_equal 2, @hotel.chefs.size
   end
 
+  def test_scope_chain_does_not_interfere_with_hmt_with_polymorphic_case_and_sti
+    @hotel = Hotel.create!
+    @hotel.mocktail_designers << MocktailDesigner.create!
+
+    assert_equal 1, @hotel.mocktail_designers.size
+    assert_equal 1, @hotel.chef_lists.size
+  end
+
   def test_scope_chain_of_polymorphic_association_does_not_leak_into_other_hmt_associations
     hotel = Hotel.create!
     department = hotel.departments.create!
diff --git a/activerecord/test/models/chef.rb b/activerecord/test/models/chef.rb
index 698a52e045..9d3dd01016 100644
--- a/activerecord/test/models/chef.rb
+++ b/activerecord/test/models/chef.rb
@@ -2,3 +2,7 @@ class Chef < ActiveRecord::Base
   belongs_to :employable, polymorphic: true
   has_many :recipes
 end
+
+class ChefList < Chef
+  belongs_to :employable_list, polymorphic: true
+end
diff --git a/activerecord/test/models/hotel.rb b/activerecord/test/models/hotel.rb
index 491f8dfde3..9c90ffcff4 100644
--- a/activerecord/test/models/hotel.rb
+++ b/activerecord/test/models/hotel.rb
@@ -3,5 +3,9 @@ class Hotel < ActiveRecord::Base
   has_many :chefs, through: :departments
   has_many :cake_designers, source_type: 'CakeDesigner', source: :employable, through: :chefs
   has_many :drink_designers, source_type: 'DrinkDesigner', source: :employable, through: :chefs
+
+  has_many :chef_lists, as: :employable_list
+  has_many :mocktail_designers, through: :chef_lists, source: :employable, :source_type => "MocktailDesigner"
+
   has_many :recipes, through: :chefs
 end
diff --git a/activerecord/test/models/mocktail_designer.rb b/activerecord/test/models/mocktail_designer.rb
new file mode 100644
index 0000000000..77b44651a3
--- /dev/null
+++ b/activerecord/test/models/mocktail_designer.rb
@@ -0,0 +1,2 @@
+class MocktailDesigner < DrinkDesigner
+end
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 2e1e584156..fcc4eee79c 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -976,6 +976,8 @@ def except(adapter_names_to_exclude)
     t.integer :employable_id
     t.string :employable_type
     t.integer :department_id
+    t.string :employable_list_type
+    t.integer :employable_list_id
   end
   create_table :recipes, force: true do |t|
     t.integer :chef_id