Active Model Attribute Methods

ActiveModel::AttributeMethods provides a way to add prefixes and suffixes to your methods as well as handling the creation of ActiveRecord::Base-like class methods such as table_name.

The requirements to implement ActiveModel::AttributeMethods are to:

  • include ActiveModel::AttributeMethods in your class.

  • Call each of its method you want to add, such as attribute_method_suffix or attribute_method_prefix.

  • Call define_attribute_methods after the other methods are called.

  • Define the various generic _attribute methods that you have declared.

  • Define an attributes method, see below.

A minimal implementation could be:

class Person
  include ActiveModel::AttributeMethods

  attribute_method_affix  prefix: 'reset_', suffix: '_to_default!'
  attribute_method_suffix '_contrived?'
  attribute_method_prefix 'clear_'
  define_attribute_methods :name

  attr_accessor :name

  def attributes
    {'name' => @name}
  end

  private

  def attribute_contrived?(attr)
    true
  end

  def clear_attribute(attr)
    send("#{attr}=", nil)
  end

  def reset_attribute_to_default!(attr)
    send("#{attr}=", 'Default Name')
  end
end

Note that whenever you include ActiveModel::AttributeMethods in your class, it requires you to implement an attributes method which returns a hash with each attribute name in your model as hash key and the attribute value as hash value.

Hash keys must be strings.

Namespace
Methods
A
M
R
Constants
NAME_COMPILABLE_REGEXP = /\A[a-zA-Z_]\w*[!?=]?\z/
 
CALL_COMPILABLE_REGEXP = /\A[a-zA-Z_]\w*[!?]?\z/
 
Instance Public methods
attribute_missing(match, *args, &block)

attribute_missing is like method_missing, but for attributes. When method_missing is called we check to see if there is a matching attribute method. If so, we tell attribute_missing to dispatch the attribute. This method can be overloaded to customize the behavior.

Source: show | on GitHub 

# File activemodel/lib/active_model/attribute_methods.rb, line 444
def attribute_missing(match, *args, &block)
  __send__(match.target, match.attr_name, *args, &block)
end
method_missing(method, *args, &block)

Allows access to the object attributes, which are held in the hash returned by attributes, as though they were first-class methods. So a Person class with a name attribute can for example use Person#name and Person#name= and never directly use the attributes hash – except for multiple assigns with ActiveRecord::Base#attributes=.

It’s also possible to instantiate related objects, so a Client class belonging to the clients table with a master_id foreign key can instantiate master through Client#master.

Source: show | on GitHub 

# File activemodel/lib/active_model/attribute_methods.rb, line 431
def method_missing(method, *args, &block)
  if respond_to_without_attributes?(method, true)
    super
  else
    match = match_attribute_method?(method.to_s)
    match ? attribute_missing(match, *args, &block) : super
  end
end
respond_to?(method, include_private_methods = false)
Also aliased as: respond_to_without_attributes?

Source: show | on GitHub 

# File activemodel/lib/active_model/attribute_methods.rb, line 452
def respond_to?(method, include_private_methods = false)
  if super
    true
  elsif !include_private_methods && super(method, true)
    # If we're here then we haven't found among non-private methods
    # but found among all methods. Which means that the given method is private.
    false
  else
    !match_attribute_method?(method.to_s).nil?
  end
end
respond_to_without_attributes?(method, include_private_methods = false)

A Person instance with a name attribute can ask person.respond_to?(:name), person.respond_to?(:name=), and person.respond_to?(:name?) which will all return true.