To include any methods on the model, use :methods.

json.rb

class Content
  def type
    self.class.name
  end
end

class TwitterContent
  def type
    "twitter"
  end
end

def show
  @contents = ...
  respond_to do |format|
    format.html
    format.json  { render :json => @contents.to_json(:methods => :type) }
  end
end