Use raw sql

db/migrate/20110125190034_unique_index_on_profile.rb

@@ -1,6 +1,7 @@
 class UniqueIndexOnProfile < ActiveRecord::Migration
   def self.up
-    columns = Profile.column_names
+    conn = ActiveRecord::Base.connection
+    columns = conn.columns("profiles").map{|c| c.name}
     ["id", "created_at", "updated_at"].each{|n| columns.delete(n)}
 
     sql = <<-SQL
@@ -8,16 +9,19 @@ class UniqueIndexOnProfile < ActiveRecord::Migration
       GROUP BY #{columns.join(',')}
         HAVING COUNT(*)>1 AND `profiles`.person_id IS NOT NULL;
     SQL
-    result = Profile.connection.execute(sql)
+    result = conn.execute(sql)
     duplicate_person_ids = result.to_a.flatten
 
     undesired_profile_ids = []
     duplicate_person_ids.each do |person_id|
-      ids = Profile.where(:person_id => person_id).map!{|p| p.id}
-      ids.pop
-      undesired_profile_id.concat(ids)
+      profile_ids = conn.execute("
+        SELECT `profiles`.id FROM `profiles`
+        WHERE `profiles`.person_id = #{person_id};").to_a.flatten
+      profile_ids.pop
+      undesired_profile_ids.concat(profile_ids)
     end
-    Profile.where(:id => undesired_profile_ids).delete_all
+    conn.execute("DELETE FROM `profiles`
+      WHERE `profiles`.id IN (#{undesired_profile_ids.join(",")});") unless undesired_profile_ids.empty?
 
     remove_index :profiles, :person_id
     add_index    :profiles, :person_id, :unique => true