Added a helper to look for an modifying migration. If one is found during :revoke, the modifying migration is deleted. If the modifying migration is not found, the creating migration is deleted.

lib/generators/active_record/devise_generator.rb

@@ -1,6 +1,7 @@
 require 'rails/generators/active_record'
 require 'generators/devise/orm_helpers'
 
+
 module ActiveRecord
   module Generators
     class DeviseGenerator < ActiveRecord::Generators::Base
@@ -11,12 +12,19 @@ module ActiveRecord
 
       def copy_devise_migration
         exists = model_exists?
-        exists = !exists if behavior == :revoke
+        unless behavior == :revoke
           unless exists
             migration_template "migration.rb", "db/migrate/devise_create_#{table_name}"
           else
             migration_template "migration_existing.rb", "db/migrate/add_devise_to_#{table_name}"
           end
+        else
+          if migration_exists?(table_name)
+            migration_template "migration_existing.rb", "db/migrate/add_devise_to_#{table_name}"
+          else
+            migration_template "migration.rb", "db/migrate/devise_create_#{table_name}"
+          end
+        end
       end
 
       def generate_model

lib/generators/devise/orm_helpers.rb

@@ -15,6 +15,14 @@ CONTENT
         File.exists?(File.join(destination_root, model_path))
       end
       
+      def migration_exists?(table_name)
+        Dir.glob("#{File.join(destination_root, migration_path)}/[0-9]*_*.rb").grep(/\d+_add_devise_to_#{table_name}.rb$/).first
+      end
+      
+      def migration_path
+        @migration_path ||= File.join("db", "migrate")
+      end
+
       def model_path
         @model_path ||= File.join("app", "models", "#{file_path}.rb")
       end

test/generators/active_record_generator_test.rb

@@ -23,6 +23,12 @@ if DEVISE_ORM == :active_record
   
     test "all files are properly deleted" do
       run_generator %w(monster)
+      run_generator %w(monster)
+      assert_migration "db/migrate/devise_create_monsters.rb"
+      assert_migration "db/migrate/add_devise_to_monsters.rb"
+      run_generator %w(monster), :behavior => :revoke
+      assert_no_migration "db/migrate/add_devise_to_monsters.rb"
+      assert_migration "db/migrate/devise_create_monsters.rb"
       run_generator %w(monster), :behavior => :revoke
       assert_no_file "app/models/monster.rb"
       assert_no_migration "db/migrate/devise_create_monsters.rb"