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: # * 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 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 # composed_of :address would add address and address=(new_address). # # 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. # # 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