Allow select with Arel and count as well as calculations with Arel

It allows a query like `User.select(:name).count` to be written
using Arel as `User.select(User.arel_table[:name]).count`.

It exposes the calculations API to accept Arel nodes:
`User.count(User.arel_table[:name])`, `User.sum(User.arel_table[:id])`,
`Account.average(Account.arel_table[:credit_limit])`,
`Account.maximum(Account.arel_table[:credit_limit])` and
`Account.minimum(Account.arel_table[:credit_limit])`.

1 files changed, 3 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index df72ba7e9c..3b7f7ca151 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -217,6 +217,8 @@ module ActiveRecord
     end
 
     def aggregate_column(column_name)
+      return column_name if Arel::Expressions === column_name
+
       if @klass.column_names.include?(column_name.to_s)
         Arel::Attribute.new(@klass.unscoped.table, column_name)
       else
@@ -366,9 +368,9 @@ module ActiveRecord
       end
     end
 
-    # TODO: refactor to allow non-string `select_values` (eg. Arel nodes).
     def select_for_count
       if select_values.present?
+        return select_values.first if select_values.one?
         select_values.join(", ")
       else
         :all