support nil as model for reform

In some cases reform is just used for coercion & validation without any persistence done.
In these cases it was advised to use nil as reform model & to mark all properties as virtual.

this works fine, however fails as .persisted? is called on nil  upon rendering in actionpack-3.2.22.2/lib/action_view/helpers/form_helper.rb:388

...
 as = options[:as]
        action, method = object.respond_to?(:persisted?) && object.persisted? ? [:edit, :put] : [:new, :post]
        options[:html].reverse_merge!(
          :class  => as ? "#{action}_#{as}" : dom_class(object, action),
          :id     => as ? "#{action}_#{as}" : [options[:namespace], dom_id(object, action)].compact.join("_").presence,
          :method => method
        )
...

lib/reform/form/active_model.rb

@@ -9,6 +9,40 @@ def self.included(base)
       extend Uber::Delegates
       delegates :model, *[:persisted?, :to_key, :to_param, :id] # Uber::Delegates
 
+      # Support nil models (aka reform without backed model)
+      def persisted?
+        if model.present?
+          super
+        else
+          false
+        end
+      end
+
+      def to_key
+        if model.present?
+          super
+        else
+          nil # see http://apidock.com/rails/ActiveModel/Conversion/to_key : nil if no ket attributes
+        end
+      end
+
+      def to_param
+        if model.present?
+          super
+        else
+          nil # see http://apidock.com/rails/ActiveModel/Conversion/to_param : nil if not persisted
+        end
+      end
+
+      def id
+        if model.present?
+          super
+        else
+          nil
+        end
+      end
+
+
       def to_model # this is called somewhere in FormBuilder and ActionController.
         self
       end

test/active_model_test.rb

@@ -138,6 +138,24 @@ class Form < Reform::Form
   end
 end
 
+class ActiveModelWithNilModel < MiniTest::Spec
+  class SongForm < Reform::Form
+    include Reform::Form::ActiveModel
+
+    property :name, virtual: true
+  end
+
+  let (:form) { SongForm.new(nil) }
+
+  it do
+    form.persisted?.must_equal false
+    form.to_key.must_equal nil
+    form.to_param.must_equal nil
+    form.to_model.must_equal form
+    form.id.must_equal nil
+    form.model_name.must_equal form.class.model_name
+  end
+end
 
 class ActiveModelWithCompositionTest < MiniTest::Spec
    class HitForm < Reform::Form