Fix `count(:all)` with eager loading and explicit select and order

This follows up ebc09ed9ad9a04338138739226a1a92c7a2707ee.

We've still experienced a regression for `size` (`count(:all)`) with
eager loading and explicit select and order when upgrading Rails to 5.1.

In that case, the eager loading enforces `distinct` to subselect but
still keep the custom select, it would cause the ORDER BY with DISTINCT
issue.

```
% ARCONN=postgresql bundle exec ruby -w -Itest test/cases/relations_test.rb -n test_size_with_eager_loading_and_custom_select_and_order
Using postgresql
Run options: -n test_size_with_eager_loading_and_custom_select_and_order --seed 8356

# Running:

E

Error:
RelationTest#test_size_with_eager_loading_and_custom_select_and_order:
ActiveRecord::StatementInvalid: PG::InvalidColumnReference: ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list
LINE 1: ..." ON "comments"."post_id" = "posts"."id" ORDER BY comments.i...
                                                             ^
```

As another problem on `distinct` is enforced, the result of `count`
becomes fewer than expected if `select` is given explicitly.

e.g.

```ruby
Post.select(:type).count
# => 11

Post.select(:type).distinct.count
# => 3
```

As long as `distinct` is enforced, we need to care to keep the result of
`count`.

This fixes both the `count` with eager loading problems.

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/test/cases/calculations_test.rb b/activerecord/test/cases/calculations_test.rb
index f1e35d6ab9..c4070fc341 100644
--- a/activerecord/test/cases/calculations_test.rb
+++ b/activerecord/test/cases/calculations_test.rb
@@ -243,6 +243,12 @@ class CalculationsTest < ActiveRecord::TestCase
     assert_queries(1) { assert_equal 11, posts.count(:all) }
   end
 
+  def test_count_with_eager_loading_and_custom_select_and_order
+    posts = Post.includes(:comments).order("comments.id").select(:type)
+    assert_queries(1) { assert_equal 11, posts.count }
+    assert_queries(1) { assert_equal 11, posts.count(:all) }
+  end
+
   def test_count_with_eager_loading_and_custom_order_and_distinct
     posts = Post.includes(:comments).order("comments.id").distinct
     assert_queries(1) { assert_equal 11, posts.count }