Get STI models working with forms (you can assign the model's type directly)

_form.html.erb

<%= f.select(:type, ['User', 'Admin']) %>

single_table_inheritance.rb

module SingleTableInheritiance
  def self.included(base)
    base.extend(SingleTableInheritiance::ClassMethods)
    base.attr_accessible(:type)
  end

  module ClassMethods
    def new(*args, &block)
      attributes = args.dup.extract_options!.with_indifferent_access
      type = attributes.delete(:type)
      if type.blank? || (type = type.constantize) == self
        super(*args, &block)
      else
        super(*args, &block).becomes(type).tap do |object|
          object.attributes = attributes
        end
      end
    end
  end
end

user.rb

class User < ActiveRecord::Base
  include SingleTableInheritance

  class Admin < User
  end
end