A super simple inline object to render a form

controller.rb

class SessionsController < ApplicationController

  def new
    @user = OpenStruct.new.tap do |model|
      def model.class
        OpenStruct.new(name: 'User').tap do |klass|
          klass.model_name = ActiveModel::Name.new(klass)
        end
      end
    end
  end

end

view.html.erb

<%= simple_form_for @user, url: new_user_session_path do |f| %>
  <%= f.input :email %>
  <%= f.input :password %>

  <%= f.button :submit, 'Sign In' %>
<% end %>

If only calling form_for @user resulted in a call to @user.model_name instead of @user.class.model_name.

The culprit is ActiveModel::Naming.model_name_from_record_or_class