提升迁移文件效率

db/migrate/20150811010817_update_activities_data.rb

@@ -1,9 +1,9 @@
 class UpdateActivitiesData < ActiveRecord::Migration
   def up 	
-  	count = Activity.all.count / 10 + 1
+  	count = Activity.all.count / 20 + 1
     transaction do
       for i in 1 ... count do i
-        Activity.all.each do |activity|
+        Activity.page(i).per(20).each do |activity|
           type = activity.act_type
           if type=='Contest' || type=='Message' || type=='News'|| type=='Journal'|| type=='Issue'|| type=='Principal'||type=='JournalsForMessage'
             activity.created_at = activity.act.created_on if activity.act