Rendering a string of Erb/HAML back to a string (render :partial from everything else but filesystem)

gistfile1.rb

# My intention is to be able to do this:
#
# from within some view, render a partial
# with the added flavour that the partial is not a file
# but rather a table model instance - like say a Customer,
# a PurchaseOrder, or in this case - a Template

# the partial rendering "should" adhere to all the standard Rails
# magic, ie nested rendering of partials, builders with Erb/HAML, 
# I18n translation support to label the most important.

# Regrettably I'm not a very good BDD/TDD'er so my tests stinks - hence 
# this elaborate example <:(

# First I write the code I'd wish I had:
#
# views/users/mailer/confirmation_instructions.haml
= build_from_template( { template: 'mail confirmation' })

# which requires me to include some Helpers
#
# mailers/users/mailer.rb
class Users::Mailer < Devise::Mailer
  default :from => 'no_reply@example.com'
  
  add_template_helper(ApplicationHelper)
  
end

# and write the function in ApplicationHelper
#
# helpers/application_helper.rb
  
  def build_from_template( options={})
    tmpl = options.delete(:template)
    tmpl = tmpl.class==String ? Template.find_by_name(tmpl) : tmpl
    tmpl.render_template( options )
  end

# I also have to do a migration which
# builds a table 'templates' and a model in models
#
# > rails generate model Template name:string, content:text, liquid_tempalte:text
#
# what eventually leads me to the model
#
# models/template.rb
class Template < ActiveRecord::Base

  def render_template(options = {})
    options.merge!( template: self, logger: Rails.logger )
    if liquid_content
      get_template.render options
    else
      Haml::Engine.new( ERB.new(content).result ).render
    end
  end

end


# - and "it just works" (except for the Rails rendering godness and routes and 
#   what not)
#   ie: basic Erb works
    "#{2+2}"                                    -> 4
    "<%= I18n.translate(:hello) %>"             -> Hello world

#       Rails stuff does not
    "<%= render partial: 'shared/errors' %>"    -> undefined method `render'
    "#{ link_to( 'test', root_path ) }"         -> undefined method `root_path'