Required methods for a custom enumerable if you want to pass it to PartialRenderer.

custom_enumerable.rb

require 'forwardable'
class CustomEnum
  extend Forwardable
  include Enumerable

  def_delegators  :@collection, :size, :each, :blank?
  attr_accessor :collection

  def initialize()
    self.collection = []
  end

  def to_ary
    self
  end
end

renderer.html.erb

...
    <%= render :partial => "warning", :collection => @foo.custom_collection %>
...