Question

Using Rails 3 and ActiveModel, I am unable to use the self. syntax to get the value of an attribute inside an ActiveModel based object.

In the following code, in save method, self.first_name evaluates to nil where @attributes[:first_name] evaluates to ‘Firstname’ (the value passed in from the controller when initializing the object).

In ActiveRecord this seems to work, but when building the same class in ActiveModel, it does not. How do you refer to a field using accessors in an ActiveModel based class?

class Card
  include ActiveModel::Validations
  extend ActiveModel::Naming 
  include ActiveModel::Conversion
  include ActiveModel::Serialization
  include ActiveModel::Serializers::Xml

  validates_presence_of :first_name

  def initialize(attributes = {})
    @attributes = attributes
  end

  #DWT TODO we need to make sure that the attributes initialize the accessors properyl, and in the same way they would if this was ActiveRecord
  attr_accessor :attributes, :first_name

  def read_attribute_for_validation(key)
    @attributes[key]
  end

  #save to the web service
  def save
    Rails.logger.info "self vs attribute:\n\t#{self.first_name}\t#{@attributes["first_name"]}"
  end

  ...

end

Answer 1

I figured it out. The “hack” that I mentioned as a comment to Marian’s answer actually turns out to be exactly how the accessors for ActiveRecord classes are generated. Here’s what I did:

class MyModel
  include ActiveModel::AttributeMethods

  attribute_method_suffix  "="  # attr_writers
  attribute_method_suffix  ""   # attr_readers

  define_attribute_methods [:foo, :bar]

  # ActiveModel expects attributes to be stored in @attributes as a hash
  attr_reader :attributes

  private

  # simulate attribute writers from method_missing
  def attribute=(attr, value)
    @attributes[attr] = value
  end

  # simulate attribute readers from method_missing
  def attribute(attr)
    @attributes[attr]
  end
end

You can see the same thing if you look in ActiveRecord’s source code (lib/active_record/attribute_methods/{read,write}.rb).