Rails equivalent to Django's "choices"

On the validation side of things, probably validates_inclusion_of is what you need:

class Coffee < ActiveRecord::Base
  validates_inclusion_of :size, :in => %w(small medium large),
    :message => "%{value} is not a valid size"
end

As for generating the helper, you can try something like:

class Coffee < ActiveRecord::Base
  @@coffe_size = %w(small medium large)

  validates_inclusion_of :size, :in => @@coffe_size,
    :message => "%{value} is not a valid size"

   def self.coffee_size_options
       @@coffe_size.map{ |z| [z,z]} 
   end
end

And then in some helper:

<%= select(:coffee, :size, Coffee.coffee_size_options) %>