module ActiveRecord
module Aggregations # :nodoc:
def self.included(base)
base.extend(ClassMethods)
end
def clear_aggregation_cache #:nodoc:
self.class.reflect_on_all_aggregations.to_a.each do |assoc|
instance_variable_set "@#{assoc.name}", nil
end unless self.new_record?
end
# Active Record implements aggregation through a macro-like class method called +composed_of+ for representing attributes
# as value objects. It expresses relationships like "Account [is] composed of Money [among other things]" or "Person [is]
# composed of [an] address". Each call to the macro adds a description of how the value objects are created from the
# attributes of the entity object (when the entity is initialized either as a new object or from finding an existing object)
# and how it can be turned back into attributes (when the entity is saved to the database). Example:
#
# class Customer < ActiveRecord::Base
# composed_of :balance, :class_name => "Money", :mapping => %w(balance amount)
# composed_of :address, :mapping => [ %w(address_street street), %w(address_city city) ]
# end
#
# The customer class now has the following methods to manipulate the value objects:
# * Customer#balance, Customer#balance=(money)
# * Customer#address, Customer#address=(address)
#
# These methods will operate with value objects like the ones described below:
#
# class Money
# include Comparable
# attr_reader :amount, :currency
# EXCHANGE_RATES = { "USD_TO_DKK" => 6 }
#
# def initialize(amount, currency = "USD")
# @amount, @currency = amount, currency
# end
#
# def exchange_to(other_currency)
# exchanged_amount = (amount * EXCHANGE_RATES["#{currency}_TO_#{other_currency}"]).floor
# Money.new(exchanged_amount, other_currency)
# end
#
# def ==(other_money)
# amount == other_money.amount && currency == other_money.currency
# end
#
# def <=>(other_money)
# if currency == other_money.currency
# amount <=> amount
# else
# amount <=> other_money.exchange_to(currency).amount
# end
# end
# end
#
# class Address
# attr_reader :street, :city
# def initialize(street, city)
# @street, @city = street, city
# end
#
# def close_to?(other_address)
# city == other_address.city
# end
#
# def ==(other_address)
# city == other_address.city && street == other_address.street
# end
# end
#
# Now it's possible to access attributes from the database through the value objects instead. If you choose to name the
# composition the same as the attribute's name, it will be the only way to access that attribute. That's the case with our
# +balance+ attribute. You interact with the value objects just like you would any other attribute, though:
#
# customer.balance = Money.new(20) # sets the Money value object and the attribute
# customer.balance # => Money value object
# customer.balance.exchanged_to("DKK") # => Money.new(120, "DKK")
# customer.balance > Money.new(10) # => true
# customer.balance == Money.new(20) # => true
# customer.balance < Money.new(5) # => false
#
# Value objects can also be composed of multiple attributes, such as the case of Address. The order of the mappings will
# determine the order of the parameters. Example:
#
# customer.address_street = "Hyancintvej"
# customer.address_city = "Copenhagen"
# customer.address # => Address.new("Hyancintvej", "Copenhagen")
# customer.address = Address.new("May Street", "Chicago")
# customer.address_street # => "May Street"
# customer.address_city # => "Chicago"
#
# == Writing value objects
#
# Value objects are immutable and interchangeable objects that represent a given value, such as a +Money+ object representing
# $5. Two +Money+ objects both representing $5 should be equal (through methods such as == and <=> from +Comparable+ if ranking
# makes sense). This is unlike entity objects where equality is determined by identity. An entity class such as +Customer+ can
# easily have two different objects that both have an address on Hyancintvej. Entity identity is determined by object or
# relational unique identifiers (such as primary keys). Normal ActiveRecord::Base classes are entity objects.
#
# It's also important to treat the value objects as immutable. Don't allow the +Money+ object to have its amount changed after
# creation. Create a new +Money+ object with the new value instead. This is exemplified by the Money#exchanged_to method that
# returns a new value object instead of changing its own values. Active Record won't persist value objects that have been
# changed through means other than the writer method.
#
# The immutable requirement is enforced by Active Record by freezing any object assigned as a value object. Attempting to
# change it afterwards will result in a TypeError.
#
# Read more about value objects on http://c2.com/cgi/wiki?ValueObject and on the dangers of not keeping value objects
# immutable on http://c2.com/cgi/wiki?ValueObjectsShouldBeImmutable
module ClassMethods
# Adds reader and writer methods for manipulating a value object:
# composed_of :address adds address and address=(new_address) methods.
#
# Options are:
# * :class_name - specify the class name of the association. Use it only if that name can't be inferred
# from the part id. So composed_of :address will by default be linked to the +Address+ class, but
# if the real class name is +CompanyAddress+, you'll have to specify it with this option.
# * :mapping - specifies a number of mapping arrays (attribute, parameter) that bind an attribute name
# to a constructor parameter on the value class.
# * :allow_nil - specifies that the aggregate object will not be instantiated when all mapped
# attributes are +nil+. Setting the aggregate class to +nil+ has the effect of writing +nil+ to all mapped attributes.
# This defaults to +false+.
#
# An optional block can be passed to convert the argument that is passed to the writer method into an instance of
# :class_name. The block will only be called if the argument is not already an instance of :class_name.
#
# Option examples:
# composed_of :temperature, :mapping => %w(reading celsius)
# composed_of(:balance, :class_name => "Money", :mapping => %w(balance amount)) {|balance| balance.to_money }
# composed_of :address, :mapping => [ %w(address_street street), %w(address_city city) ]
# composed_of :gps_location
# composed_of :gps_location, :allow_nil => true
#
def composed_of(part_id, options = {}, &block)
options.assert_valid_keys(:class_name, :mapping, :allow_nil)
name = part_id.id2name
class_name = options[:class_name] || name.camelize
mapping = options[:mapping] || [ name, name ]
mapping = [ mapping ] unless mapping.first.is_a?(Array)
allow_nil = options[:allow_nil] || false
reader_method(name, class_name, mapping, allow_nil)
writer_method(name, class_name, mapping, allow_nil, block)
create_reflection(:composed_of, part_id, options, self)
end
private
def reader_method(name, class_name, mapping, allow_nil)
module_eval do
define_method(name) do |*args|
force_reload = args.first || false
if (instance_variable_get("@#{name}").nil? || force_reload) && (!allow_nil || mapping.any? {|pair| !read_attribute(pair.first).nil? })
instance_variable_set("@#{name}", class_name.constantize.new(*mapping.collect {|pair| read_attribute(pair.first)}))
end
instance_variable_get("@#{name}")
end
end
end
def writer_method(name, class_name, mapping, allow_nil, conversion)
module_eval do
define_method("#{name}=") do |part|
if part.nil? && allow_nil
mapping.each { |pair| @attributes[pair.first] = nil }
instance_variable_set("@#{name}", nil)
else
part = conversion.call(part) unless part.is_a?(class_name.constantize) || conversion.nil?
mapping.each { |pair| @attributes[pair.first] = part.send(pair.last) }
instance_variable_set("@#{name}", part.freeze)
end
end
end
end
end
end
end