Only run the data migration if there are profiles in the db

db/migrate/20110125190034_unique_index_on_profile.rb

@@ -1,27 +1,30 @@
 class UniqueIndexOnProfile < ActiveRecord::Migration
+  class Profile < ActiveRecord::Base; end
   def self.up
-    conn = ActiveRecord::Base.connection
-    columns = conn.columns("profiles").map{|c| c.name}
-    ["id", "created_at", "updated_at"].each{|n| columns.delete(n)}
+    if Profile.count > 0
+      conn = ActiveRecord::Base.connection
+      columns = conn.columns("profiles").map{|c| c.name}
+      ["id", "created_at", "updated_at"].each{|n| columns.delete(n)}
 
-    sql = <<-SQL
-    SELECT profiles.person_id FROM profiles
-      GROUP BY #{columns.join(',')}
-        HAVING COUNT(*)>1 AND profiles.person_id IS NOT NULL;
-    SQL
-    result = conn.execute(sql)
-    duplicate_person_ids = result.to_a.flatten
+      sql = <<-SQL
+      SELECT profiles.person_id FROM profiles
+        GROUP BY #{columns.join(',')}
+          HAVING COUNT(*)>1 AND profiles.person_id IS NOT NULL;
+      SQL
+      result = conn.execute(sql)
+      duplicate_person_ids = result.to_a.flatten
 
-    undesired_profile_ids = []
-    duplicate_person_ids.each do |person_id|
-      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)
+      undesired_profile_ids = []
+      duplicate_person_ids.each do |person_id|
+        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
+      conn.execute("DELETE FROM profiles
+        WHERE profiles.id IN (#{undesired_profile_ids.join(",")});") unless undesired_profile_ids.empty?
     end
-    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
@@ -29,5 +32,6 @@ class UniqueIndexOnProfile < ActiveRecord::Migration
 
   def self.down
     remove_index :profiles, :person_id
+    add_index    :profiles, :person_id
   end
 end