Initializer caveats warning for Mongoid < 2.1

lib/devise/rails.rb

@@ -39,5 +39,18 @@ class Engine < ::Rails::Engine
         Devise.include_helpers(Devise::OmniAuth)
       end
     end
+
+    initializer "devise.mongoid_version_warning" do
+      if defined?(Mongoid)
+        require 'mongoid/version'
+        if Mongoid::VERSION.to_f < 2.1
+          puts "\n[DEVISE] Please note that Mongoid versions prior to 2.1 handle dirty model " \
+            "object attributes in such a way that the Devise `validatable` module will not apply " \
+            "its usual uniqueness and format validations for the email field. It is recommended " \
+            "that you upgrade to Mongoid 2.1+ for this and other fixes, but if for some reason you " \
+            "are unable to do so, you should add these validations manually.\n"
+        end
+      end
+    end
   end
 end