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module ActiveRecord
module Aggregations # :nodoc:
def self.append_features(base)
super
base.extend(ClassMethods)
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 on 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)
# 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:
# * <tt>Customer#balance, Customer#balance=(money)</tt>
# * <tt>Customer#address, Customer#address=(address)</tt>
#
# 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 attributes 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 == and <=> from Comparable if ranking makes
# sense). This is unlike a 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 other means 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 the a reader and writer method for manipulating a value object, so
# <tt>composed_of :address</tt> would add <tt>address</tt> and <tt>address=(new_address)</tt>.
#
# Options are:
# * <tt>:class_name</tt> - specify the class name of the association. Use it only if that name can't be inferred
# from the part id. So <tt>composed_of :address</tt> 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.
# * <tt>:mapping</tt> - specifies a number of mapping arrays (attribute, parameter) that bind an attribute name
# to a constructor parameter on the value class.
#
# Option examples:
# composed_of :temperature, :mapping => %w(reading celsius)
# composed_of :balance, :class_name => "Money", :mapping => %w(balance amount)
# composed_of :address, :mapping => [ %w(address_street street), %w(address_city city) ]
# composed_of :gps_location
def composed_of(part_id, options = {})
options.assert_valid_keys(:class_name, :mapping)
name = part_id.id2name
class_name = options[:class_name] || name_to_class_name(name)
mapping = options[:mapping] || [ name, name ]
reader_method(name, class_name, mapping)
writer_method(name, class_name, mapping)
end
private
def name_to_class_name(name)
name.capitalize.gsub(/_(.)/) { |s| $1.capitalize }
end
def reader_method(name, class_name, mapping)
module_eval <<-end_eval
def #{name}(force_reload = false)
if @#{name}.nil? || force_reload
@#{name} = #{class_name}.new(#{(Array === mapping.first ? mapping : [ mapping ]).collect{ |pair| "read_attribute(\"#{pair.first}\")"}.join(", ")})
end
return @#{name}
end
end_eval
end
def writer_method(name, class_name, mapping)
module_eval <<-end_eval
def #{name}=(part)
@#{name} = part.freeze
#{(Array === mapping.first ? mapping : [ mapping ]).collect{ |pair| "@attributes[\"#{pair.first}\"] = part.#{pair.last}" }.join("\n")}
end
end_eval
end
end
end
end
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