removing duplicate tag followings

db/migrate/20111026173547_add_missing_tag_followings_indices.rb

@@ -1,5 +1,26 @@
 class AddMissingTagFollowingsIndices < ActiveRecord::Migration
+  def self.delete_duplicate_tag_followings
+    duplicate_rows = execute <<SQL
+      SELECT COUNT(tf.user_id), tf.user_id, tf.tag_id from tag_followings AS tf
+        GROUP BY tf.user_id, tf.tag_id
+          HAVING COUNT(*)>1;
+SQL
+    duplicate_rows.each do |row|
+      count = row.first
+      user_id = row[1]
+      tag_id = row.last
+
+      execute <<SQL
+        DELETE FROM tag_followings
+        WHERE tag_followings.user_id = #{user_id} AND tag_followings.tag_id = #{tag_id}
+        LIMIT #{count-1}
+SQL
+    end
+  end
+
   def self.up
+    delete_duplicate_tag_followings
+
     add_index :tag_followings, :tag_id
     add_index :tag_followings, :user_id
     add_index :tag_followings, [:tag_id, :user_id], :unique => true