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, 5 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index 0384023a17..29b1dc97b7 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -129,11 +129,12 @@ module ActiveRecord
         relation = apply_join_dependency
 
         if operation.to_s.downcase == "count"
-          relation.distinct!
-          # PostgreSQL: ORDER BY expressions must appear in SELECT list when using DISTINCT
-          if (column_name == :all || column_name.nil?) && select_values.empty?
-            relation.order_values = []
+          unless distinct_value || distinct_select?(column_name || select_for_count)
+            relation.distinct!
+            relation.select_values = [ klass.primary_key || table[Arel.star] ]
           end
+          # PostgreSQL: ORDER BY expressions must appear in SELECT list when using DISTINCT
+          relation.order_values = []
         end
 
         relation.calculate(operation, column_name)