Fix generated projection fields in group by query

Closes #21922

Let `Book(id, author_id)`, `Photo(id, book_id, author_id)` and `Author(id)`

Running `Book.group(:author_id).joins(:photos).count` will produce:

* Rails 4.2 - conflicts `author_id` in both projection and group by:
```sql
SELECT COUNT(*) AS count_all, author_id AS author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY author_id
```

* Master (9d02a25) - conflicts `author_id` only in projection:
```sql
SELECT COUNT(*) AS count_all, author_id AS author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY "books"."author_id"
```

* With this fix:
```sql
SELECT COUNT(*) AS count_all, "books"."author_id" AS books_author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY "books"."author_id"
```

3 files changed, 14 insertions, 1 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 258b2be899..e1f543c3a1 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Queries such as `Computer.joins(:monitor).group(:status).count` will now be
+    interpreted as  `Computer.joins(:monitor).group('computers.status').count`
+    so that when `Computer` and `Monitor` have both `status` columns we don't
+    have conflicts in projection.
+
+    *Rafael Sales*
+
 *   Add ability to default to `uuid` as primary key when generating database migrations
 
     Set `Rails.application.config.active_record.primary_key = :uuid`
diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index 8b9367acc7..6d3e5327fb 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -275,6 +275,7 @@ module ActiveRecord
       else
         group_fields = group_attrs
       end
+      group_fields = arel_columns(group_fields)
 
       group_aliases = group_fields.map { |field|
         column_alias_for(field)
@@ -299,7 +300,7 @@ module ActiveRecord
       ]
       select_values += select_values unless having_clause.empty?
 
-      select_values.concat arel_columns(group_fields).zip(group_aliases).map { |field,aliaz|
+      select_values.concat group_fields.zip(group_aliases).map { |field,aliaz|
         if field.respond_to?(:as)
           field.as(aliaz)
         else
diff --git a/activerecord/test/cases/calculations_test.rb b/activerecord/test/cases/calculations_test.rb
index a0e4b4971b..4a0e6f497f 100644
--- a/activerecord/test/cases/calculations_test.rb
+++ b/activerecord/test/cases/calculations_test.rb
@@ -455,6 +455,11 @@ class CalculationsTest < ActiveRecord::TestCase
     [1,6,2,9].each { |firm_id| assert c.keys.include?(firm_id) }
   end
 
+  def test_should_count_field_of_root_table_with_conflicting_group_by_column
+    assert_equal({ 1 => 1 }, Firm.joins(:accounts).group(:firm_id).count)
+    assert_equal({ 1 => 1 }, Firm.joins(:accounts).group('accounts.firm_id').count)
+  end
+
   def test_count_with_no_parameters_isnt_deprecated
     assert_not_deprecated { Account.count }
   end