MySQL: You can't specify target table for update in FROM clause (#23347).

Patch by Felix Schäfer.

git-svn-id: http://svn.redmine.org/redmine/trunk@15737 e93f8b46-1217-0410-a6f0-8f06a7374b81

db/migrate/20150525103953_clear_estimated_hours_on_parent_issues.rb

@@ -6,7 +6,7 @@ class ClearEstimatedHoursOnParentIssues < ActiveRecord::Migration
 
   def self.down
     table_name = Issue.table_name
-    leaves_sum_select = "SELECT SUM(leaves.estimated_hours) FROM #{table_name} leaves" +
+    leaves_sum_select = "SELECT SUM(leaves.estimated_hours) FROM (SELECT * FROM #{table_name}) AS leaves" +
       " WHERE leaves.root_id = #{table_name}.root_id AND leaves.lft > #{table_name}.lft AND leaves.rgt < #{table_name}.rgt" +
       " AND leaves.rgt = leaves.lft + 1"