open the ContactFormController to model subclasses

app/controllers/hyrax/contact_form_controller.rb

@@ -4,6 +4,9 @@ class ContactFormController < ApplicationController
     before_action :build_contact_form
     layout 'homepage'
 
+    class_attribute :model_class
+    self.model_class = Hyrax::ContactForm
+
     def new; end
 
     def create
@@ -12,7 +15,7 @@ def create
         ContactMailer.contact(@contact_form).deliver_now
         flash.now[:notice] = 'Thank you for your message!'
         after_deliver
-        @contact_form = ContactForm.new
+        @contact_form = model_class.new
       else
         flash.now[:error] = 'Sorry, this message was not sent successfully. ' +
                             @contact_form.errors.full_messages.map(&:to_s).join(", ")
@@ -36,7 +39,7 @@ def after_deliver; end
     private
 
     def build_contact_form
-      @contact_form = Hyrax::ContactForm.new(contact_form_params)
+      @contact_form = model_class.new(contact_form_params)
     end
 
     def contact_form_params

spec/controllers/hyrax/contact_form_controller_spec.rb

@@ -13,6 +13,22 @@
 
   before { sign_in(user) }
 
+  describe '.model_class' do
+    let(:fake_contact_form_class) { Class.new(Hyrax::ContactForm) }
+
+    it 'is Hyrax::ContactForm by default' do
+      expect(described_class.model_class).to eq Hyrax::ContactForm
+    end
+
+    it 'builds an instance of the class for assigns' do
+      allow(described_class).to receive(:model_class).and_return(fake_contact_form_class)
+
+      get :new
+
+      expect(assigns[:contact_form]).to be_a fake_contact_form_class
+    end
+  end
+
   describe "#new" do
     subject { response }