Specs surely don't pass, but they run at least!

test/orm/data_mapper.rb

@@ -1 +1,10 @@
+require 'rails/test_help'
+
 DataMapper.auto_migrate!
+
+class ActiveSupport::TestCase
+  setup do
+    User.all.destroy!
+    Admin.all.destroy!
+  end
+end

test/rails_app/app/data_mapper/shim.rb

@@ -0,0 +1,2 @@
+module Shim
+end

test/rails_app/app/data_mapper/user.rb

@@ -1,6 +1,9 @@
 class User
   include DataMapper::Resource
-  
+
+  extend  Devise::Orm::DataMapper::Schema
+  include Devise::Orm::DataMapper::Compatibility
+
   property :id, Serial
   property :username, String