Fix reset of the source association when through association is loaded

The special case happens when through association has a custom scope
that is applied to the source association when loading.
In this case, the soucre association would need to be reset after
main association is loaded. See tests.

The special case exists when a through association has

5 files changed, 22 insertions, 24 deletions

diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index d6f7359055..85a994b27a 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -116,7 +116,7 @@ module ActiveRecord
           def build_scope
             scope = klass.scope_for_association
 
-            if reflection.type
+            if reflection.type && !reflection.through_reflection?
               scope.where!(reflection.type => model.polymorphic_name)
             end
 
diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index a6b7ab80a2..25254e652a 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -7,10 +7,9 @@ module ActiveRecord
         def run(preloader)
           already_loaded     = owners.first.association(through_reflection.name).loaded?
           through_scope      = through_scope()
-          reflection_scope   = target_reflection_scope
           through_preloaders = preloader.preload(owners, through_reflection.name, through_scope)
           middle_records     = through_preloaders.flat_map(&:preloaded_records)
-          preloaders         = preloader.preload(middle_records, source_reflection.name, reflection_scope)
+          preloaders         = preloader.preload(middle_records, source_reflection.name, scope)
           @preloaded_records = preloaders.flat_map(&:preloaded_records)
 
           owners.each do |owner|
@@ -25,18 +24,18 @@ module ActiveRecord
               owner.association(through_reflection.name).reset if through_scope
             end
             result = through_records.flat_map do |record|
-              association = record.association(source_reflection.name)
-              target = association.target
-              association.reset if preload_scope
-              target
+              record.association(source_reflection.name).target
             end
             result.compact!
-            if reflection_scope
-              result.sort_by! { |rhs| preload_index[rhs] } if reflection_scope.order_values.any?
-              result.uniq! if reflection_scope.distinct_value
-            end
+            result.sort_by! { |rhs| preload_index[rhs] } if scope.order_values.any?
+            result.uniq! if scope.distinct_value
             associate_records_to_owner(owner, result)
           end
+          unless scope.empty_scope?
+            middle_records.each do |owner|
+              owner.association(source_reflection.name).reset
+            end
+          end
         end
 
         private
@@ -91,16 +90,6 @@ module ActiveRecord
 
             scope unless scope.empty_scope?
           end
-
-          def target_reflection_scope
-            if preload_scope
-              reflection_scope.merge(preload_scope)
-            elsif reflection.scope
-              reflection_scope
-            else
-              nil
-            end
-          end
       end
     end
   end
diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index 5fdc5a92fc..38e25a9100 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -2080,10 +2080,12 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
   end
 
   def test_associations_order_should_be_priority_over_throughs_order
-    david = authors(:david)
+    original = authors(:david)
     expected = [12, 10, 9, 8, 7, 6, 5, 3, 2, 1]
-    assert_equal expected, david.comments_desc.map(&:id)
-    assert_equal expected, Author.includes(:comments_desc).find(david.id).comments_desc.map(&:id)
+    assert_equal expected, original.comments_desc.map(&:id)
+    preloaded = Author.includes(:comments_desc).find(original.id)
+    assert_equal expected, preloaded.comments_desc.map(&:id)
+    assert_equal original.posts_sorted_by_id.first.comments.map(&:id), preloaded.posts_sorted_by_id.first.comments.map(&:id)
   end
 
   def test_dynamic_find_should_respect_association_order_for_through
diff --git a/activerecord/test/cases/associations/nested_through_associations_test.rb b/activerecord/test/cases/associations/nested_through_associations_test.rb
index 5821744530..0b83fd8421 100644
--- a/activerecord/test/cases/associations/nested_through_associations_test.rb
+++ b/activerecord/test/cases/associations/nested_through_associations_test.rb
@@ -610,6 +610,12 @@ class NestedThroughAssociationsTest < ActiveRecord::TestCase
     assert_equal hotel, Hotel.joins(:cake_designers, :drink_designers).take
   end
 
+  def test_has_many_through_reset_source_reflection_after_loading_is_complete
+    preloaded = Category.preload(:ordered_post_comments).find(1, 2).last
+    original = Category.find(2)
+    assert_equal original.ordered_post_comments.ids, preloaded.ordered_post_comments.ids
+  end
+
   private
 
     def assert_includes_and_joins_equal(query, expected, association)
diff --git a/activerecord/test/models/category.rb b/activerecord/test/models/category.rb
index 2ccc00bed9..8c86879dc6 100644
--- a/activerecord/test/models/category.rb
+++ b/activerecord/test/models/category.rb
@@ -26,6 +26,7 @@ class Category < ActiveRecord::Base
   has_many :categorizations
   has_many :special_categorizations
   has_many :post_comments, through: :posts, source: :comments
+  has_many :ordered_post_comments, -> { order(id: :desc) }, through: :posts, source: :comments
 
   has_many :authors, through: :categorizations
   has_many :authors_with_select, -> { select "authors.*, categorizations.post_id" }, through: :categorizations, source: :author