delete dup pvs in migration

db/migrate/20110211204804_unique_index_post_visibilities.rb

 class UniqueIndexPostVisibilities < ActiveRecord::Migration
   def self.up
+    sql = <<-SQL
+    SELECT `post_visibilities`.post_id, `post_visibilities`.aspect_id FROM `post_visibilities`
+      GROUP BY post_id, aspect_id
+        HAVING COUNT(*)>1;
+    SQL
+
+    result = execute(sql)
+    dup_pvs = result.to_a
+    undesired_ids = []
+
+    dup_pvs.each do |arr|
+      post_id, aspect_id = arr
+      pv_ids = execute("
+        SELECT `post_visibilities`.id FROM `post_visibilities`
+        WHERE `post_visibilities`.post_id = #{post_id}
+          AND `post_visibilities`.aspect_id = #{aspect_id};"
+      )
+      pv_ids.pop
+      undesired_ids.concat(pv_ids)
+    end
+    execute("DELETE FROM `post_visibilities` WHERE `post_visibiilties`.id IN (#{undesired_ids.join(",")});") unless undesired_ids.empty?
+
+
     remove_index :post_visibilities, [:aspect_id, :post_id]
     add_index :post_visibilities, [:aspect_id, :post_id], :unique => true
   end