Fix assigning through records when using polymorphic has many through association

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -59,9 +59,10 @@ def build_through_record(record)
 
             attributes = through_scope_attributes
             attributes[source_reflection.name] = record
-            attributes[source_reflection.foreign_type] = options[:source_type] if options[:source_type]
 
-            through_association.build(attributes)
+            through_association.build(attributes).tap do |new_record|
+              new_record.send("#{source_reflection.foreign_type}=", options[:source_type]) if options[:source_type]
+            end
           end
         end
 

activerecord/test/cases/reflection_test.rb

@@ -27,6 +27,7 @@
 require "models/drink_designer"
 require "models/recipe"
 require "models/user_with_invalid_relation"
+require "models/hardback"
 
 class ReflectionTest < ActiveRecord::TestCase
   include ActiveRecord::Reflection
@@ -324,7 +325,7 @@ def test_scope_chain_does_not_interfere_with_hmt_with_polymorphic_case
     assert_equal 2, hotel.chefs.count
   end
 
-  def test_scope_chain_does_not_interfere_with_hmt_with_polymorphic_case_and_sti
+  def test_scope_chain_does_not_interfere_with_hmt_with_polymorphic_case_and_subclass_source
     hotel = Hotel.create!
     hotel.mocktail_designers << MocktailDesigner.create!
 
@@ -341,6 +342,20 @@ def test_scope_chain_does_not_interfere_with_hmt_with_polymorphic_case_and_sti
     assert_equal 0, hotel.chef_lists.count
   end
 
+  def test_scope_chain_does_not_interfere_with_hmt_with_polymorphic_and_subclass_source_2
+    author = Author.create!(name: "John Doe")
+    hardback = BestHardback.create!
+    author.best_hardbacks << hardback
+
+    assert_equal [hardback], author.best_hardbacks
+    assert_equal [hardback], author.reload.best_hardbacks
+
+    author.best_hardbacks = []
+
+    assert_empty author.best_hardbacks
+    assert_empty author.reload.best_hardbacks
+  end
+
   def test_scope_chain_of_polymorphic_association_does_not_leak_into_other_hmt_associations
     hotel = Hotel.create!
     department = hotel.departments.create!

activerecord/test/models/author.rb

@@ -168,6 +168,7 @@ def ratings
   has_many :tags_with_primary_key, through: :posts
 
   has_many :books
+  has_many :best_hardbacks, through: :books, source: :format_record, source_type: "BestHardback"
   has_many :published_books, class_name: "PublishedBook"
   has_many :unpublished_books, -> { where(status: [:proposed, :written]) }, class_name: "Book"
   has_many :subscriptions,        through: :books

activerecord/test/models/book.rb

@@ -2,6 +2,7 @@
 
 class Book < ActiveRecord::Base
   belongs_to :author
+  belongs_to :format_record, polymorphic: true
 
   has_many :citations, foreign_key: "book1_id", inverse_of: :book
   has_many :references, -> { distinct }, through: :citations, source: :reference_of

activerecord/test/models/hardback.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class Hardback < ActiveRecord::Base
+end
+
+class BestHardback < Hardback
+end

activerecord/test/schema/schema.rb

@@ -125,6 +125,8 @@
     default_zero = { default: 0 }
     t.references :author
     t.string :format
+    t.integer :format_record_id
+    t.string :format_record_type
     t.column :name, :string
     t.column :status, :integer, **default_zero
     t.column :last_read, :integer, **default_zero
@@ -165,6 +167,9 @@
     t.datetime :updated_at
   end
 
+  create_table :hardbacks, force: true do |t|
+  end
+
   create_table :booleans, force: true do |t|
     t.boolean :value
     t.boolean :has_fun, null: false, default: false