Merge branch 'rails5-fix-mysql-arel-from' into 'master'

Rails5 fix arel from in mysql_median_datetime_sql

See merge request gitlab-org/gitlab-ce!20167

changelogs/unreleased/rails5-fix-mysql-arel-from.yml

+---
+title: Rails5 fix arel from in mysql_median_datetime_sql
+merge_request: 20167
+author: Jasper Maes
+type: fixed

lib/gitlab/database/median.rb

@@ -33,7 +33,13 @@ module Gitlab
       end
 
       def mysql_median_datetime_sql(arel_table, query_so_far, column_sym)
-        query = arel_table
+        arel_from = if Gitlab.rails5?
+                      arel_table.from
+                    else
+                      arel_table
+                    end
+
+        query = arel_from
                 .from(arel_table.project(Arel.sql('*')).order(arel_table[column_sym]).as(arel_table.table_name))
                 .project(average([arel_table[column_sym]], 'median'))
                 .where(