迁移优化，原来需要1800秒，现只需800秒

db/migrate/20150227085333_add_data_to_project_issues_indexs_in_issue.rb

@@ -1,9 +1,11 @@
 class AddDataToProjectIssuesIndexsInIssue < ActiveRecord::Migration
   def change
-    for i in 1 ... 1000 do i
-      Issue.page(i).per(10).each do |e|
-        index = e.project.issues.index(e).to_i + 1
-        execute("update issues set project_issues_index = #{index} where id = #{e.id}")
+    transaction do
+      for i in 1 ... 1000 do i
+        Issue.page(i).per(10).each do |e|
+          index = e.project.issues.index(e).to_i + 1
+          execute("update issues set project_issues_index = #{index} where id = #{e.id}")
+        end
       end
     end
   end