Merge pull request rails#44843 from schlubbi/schlubbi/has-many-through-transactions

Use the through_reflection class to open transactions

Author: eileencodes

activerecord/lib/active_record/associations/through_association.rb

@@ -7,6 +7,10 @@ module ThroughAssociation # :nodoc:
       delegate :source_reflection, to: :reflection
 
       private
+        def transaction(&block)
+          through_reflection.klass.transaction(&block)
+        end
+
         def through_reflection
           @through_reflection ||= begin
             refl = reflection.through_reflection

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -892,6 +892,16 @@ def test_association_proxy_transaction_method_starts_transaction_in_association_
     end
   end
 
+  def test_has_many_through_uses_the_through_model_to_create_transactions
+    post   = posts(:thinking)
+    person = people(:david)
+    other_person = people(:michael)
+
+    assert_called(Reader, :transaction) do
+      post.people = [person, other_person]
+    end
+  end
+
   def test_has_many_association_through_a_belongs_to_association_where_the_association_doesnt_exist
     post = Post.create!(title: "TITLE", body: "BODY")
     assert_equal [], post.author_favorites