Revert "Merge pull request #22486 from methyl/fix-includes-for-groupped-association"

This reverts commit 537ac7d6ade61e95f2b70685ff2236b7de965bab, reversing
changes made to 9c9c54abe08d86967efd3dcac1d65158a0ff74ea.

Reason:
The way we preload associations will change the meaning of GROUP BY
operations. This is illustrated in the SQL generated by the added
test (failing on PG):

Association Load:
D, [2015-12-21T12:26:07.169920 #26969] DEBUG -- :   Post Load (0.7ms)  SELECT "posts".* FROM "posts" LEFT JOIN comments ON comments.post_id = posts.id WHERE "posts"."author_id" = $1 GROUP BY posts.id ORDER BY SUM(comments.tags_count)  [["author_id", 1]]

Preload:
D, [2015-12-21T12:26:07.128305 #26969] DEBUG -- :   Post Load (1.3ms)  SELECT "posts".* FROM "posts" LEFT JOIN comments ON comments.post_id = posts.id WHERE "posts"."author_id" IN (1, 2, 3) GROUP BY posts.id ORDER BY SUM(comments.tags_count)

1 files changed, 0 insertions, 17 deletions

diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index 966789884b..874d53c51f 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -1242,23 +1242,6 @@ class EagerAssociationTest < ActiveRecord::TestCase
     assert_equal projects.last.mentor.developers.first.contracts, projects.last.developers.last.contracts
   end
 
-  def test_eager_load_with_group_clause
-    assert_nothing_raised(ActiveRecord::StatementInvalid) do
-      subclass = Class.new(ActiveRecord::Base) do
-        def self.name; "Author"; end
-        self.table_name = "authors"
-        has_many :posts_ordered_by_comments_tags_count, -> {
-          joins('LEFT JOIN comments ON comments.post_id = posts.id').
-          order("SUM(comments.tags_count)").
-          group('posts.id') }, class_name: "Post"
-      end
-
-      preloaded = subclass.includes(:posts_ordered_by_comments_tags_count).map(&:posts_ordered_by_comments_tags_count)
-      loaded = subclass.all.map(&:posts_ordered_by_comments_tags_count)
-      assert preloaded == loaded, 'Preloaded records differ from expected result.'
-    end
-  end
-
   test "scoping with a circular preload" do
     assert_equal Comment.find(1), Comment.preload(:post => :comments).scoping { Comment.find(1) }
   end