gistfile1.rb

require 'rubygems'
require 'sinatra/base'
require 'padrino-helpers'
require 'padrino-core/application/rendering'
require 'erubis'

class SimpleApp < Sinatra::Base
  include Padrino::Helpers
  include Padrino::Rendering

  get "/" do
    erb :demo
  end
end

SimpleApp.run!

In demo you are expected to render a partial. When I include the rendering module, this works. When I don't I get the template engine exception.

As long as you either

a) include the rendering module as shown here for Padrino 0.9.26 (which simply makes the render method more powerful)
b) use an explicit engine as shown partial("foo", :engine => :erb) in edge padrino after this commit right now

Note that b only works after changes I just made in Padrino so you must work off git because in edge padrino, I also added another way to get partials working without the need for augmenting rendering:

https://gist.github.com/956825

Option A works with stable gem Padrino. By adding rendering module you can also do things like this:

render 'foo' # rather than erb :foo

without an engine and Sinatra will autodetect that for you along with smarter rendering overall.