Fix preloader to never reset associations in case they are already loaded

This patch fixes the issue when association is preloaded with a custom
preload scope which disposes the already preloaded target of the
association by reseting it.

When custom preload scope is used, the preloading is now performed into
a separated Hash - #records_by_owner instead of the association.
It removes the necessaty the reset the association after the preloading
is complete so that reset of the preloaded association never happens.

Preloading is still happening to the association when the preload scope
is empty.

1 files changed, 9 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/nested_through_associations_test.rb b/activerecord/test/cases/associations/nested_through_associations_test.rb
index 0b83fd8421..35da74102d 100644
--- a/activerecord/test/cases/associations/nested_through_associations_test.rb
+++ b/activerecord/test/cases/associations/nested_through_associations_test.rb
@@ -548,6 +548,15 @@ class NestedThroughAssociationsTest < ActiveRecord::TestCase
     end
   end
 
+  def test_through_association_preload_doesnt_reset_source_association_if_already_preloaded
+    blue = tags(:blue)
+    authors = Author.preload(posts: :first_blue_tags_2, misc_post_first_blue_tags_2: {}).to_a.sort_by(&:id)
+
+    assert_no_queries do
+      assert_equal [blue], authors[2].posts.first.first_blue_tags_2
+    end
+  end
+
   def test_nested_has_many_through_with_conditions_on_source_associations_preload_via_joins
     # Pointless condition to force single-query loading
     assert_includes_and_joins_equal(