--- /dev/null
+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:
+ # * <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 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.exchange_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 <tt>==</tt> and <tt><=></tt> 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#exchange_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 ActiveSupport::FrozenObjectError.
+ #
+ # 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
+ #
+ # == Custom constructors and converters
+ #
+ # By default value objects are initialized by calling the <tt>new</tt> constructor of the value class passing each of the
+ # mapped attributes, in the order specified by the <tt>:mapping</tt> option, as arguments. If the value class doesn't support
+ # this convention then +composed_of+ allows a custom constructor to be specified.
+ #
+ # When a new value is assigned to the value object the default assumption is that the new value is an instance of the value
+ # class. Specifying a custom converter allows the new value to be automatically converted to an instance of value class if
+ # necessary.
+ #
+ # For example, the NetworkResource model has +network_address+ and +cidr_range+ attributes that should be aggregated using the
+ # NetAddr::CIDR value class (http://netaddr.rubyforge.org). The constructor for the value class is called +create+ and it
+ # expects a CIDR address string as a parameter. New values can be assigned to the value object using either another
+ # NetAddr::CIDR object, a string or an array. The <tt>:constructor</tt> and <tt>:converter</tt> options can be used to
+ # meet these requirements:
+ #
+ # class NetworkResource < ActiveRecord::Base
+ # composed_of :cidr,
+ # :class_name => 'NetAddr::CIDR',
+ # :mapping => [ %w(network_address network), %w(cidr_range bits) ],
+ # :allow_nil => true,
+ # :constructor => Proc.new { |network_address, cidr_range| NetAddr::CIDR.create("#{network_address}/#{cidr_range}") },
+ # :converter => Proc.new { |value| NetAddr::CIDR.create(value.is_a?(Array) ? value.join('/') : value) }
+ # end
+ #
+ # # This calls the :constructor
+ # network_resource = NetworkResource.new(:network_address => '192.168.0.1', :cidr_range => 24)
+ #
+ # # These assignments will both use the :converter
+ # network_resource.cidr = [ '192.168.2.1', 8 ]
+ # network_resource.cidr = '192.168.0.1/24'
+ #
+ # # This assignment won't use the :converter as the value is already an instance of the value class
+ # network_resource.cidr = NetAddr::CIDR.create('192.168.2.1/8')
+ #
+ # # Saving and then reloading will use the :constructor on reload
+ # network_resource.save
+ # network_resource.reload
+ #
+ # == Finding records by a value object
+ #
+ # Once a +composed_of+ relationship is specified for a model, records can be loaded from the database by specifying an instance
+ # of the value object in the conditions hash. The following example finds all customers with +balance_amount+ equal to 20 and
+ # +balance_currency+ equal to "USD":
+ #
+ # Customer.find(:all, :conditions => {:balance => Money.new(20, "USD")})
+ #
+ module ClassMethods
+ # Adds reader and writer methods for manipulating a value object:
+ # <tt>composed_of :address</tt> adds <tt>address</tt> and <tt>address=(new_address)</tt> methods.
+ #
+ # Options are:
+ # * <tt>:class_name</tt> - Specifies 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 the mapping of entity attributes to attributes of the value object. Each mapping
+ # is represented as an array where the first item is the name of the entity attribute and the second item is the
+ # name the attribute in the value object. The order in which mappings are defined determine the order in which
+ # attributes are sent to the value class constructor.
+ # * <tt>:allow_nil</tt> - Specifies that the value object will not be instantiated when all mapped
+ # attributes are +nil+. Setting the value object to +nil+ has the effect of writing +nil+ to all mapped attributes.
+ # This defaults to +false+.
+ # * <tt>:constructor</tt> - A symbol specifying the name of the constructor method or a Proc that is called to
+ # initialize the value object. The constructor is passed all of the mapped attributes, in the order that they
+ # are defined in the <tt>:mapping option</tt>, as arguments and uses them to instantiate a <tt>:class_name</tt> object.
+ # The default is <tt>:new</tt>.
+ # * <tt>:converter</tt> - A symbol specifying the name of a class method of <tt>:class_name</tt> or a Proc that is
+ # called when a new value is assigned to the value object. The converter is passed the single value that is used
+ # in the assignment and is only called if the new value is not an instance of <tt>:class_name</tt>.
+ #
+ # Option examples:
+ # composed_of :temperature, :mapping => %w(reading celsius)
+ # composed_of :balance, :class_name => "Money", :mapping => %w(balance amount), :converter => Proc.new { |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
+ # composed_of :ip_address,
+ # :class_name => 'IPAddr',
+ # :mapping => %w(ip to_i),
+ # :constructor => Proc.new { |ip| IPAddr.new(ip, Socket::AF_INET) },
+ # :converter => Proc.new { |ip| ip.is_a?(Integer) ? IPAddr.new(ip, Socket::AF_INET) : IPAddr.new(ip.to_s) }
+ #
+ def composed_of(part_id, options = {}, &block)
+ options.assert_valid_keys(:class_name, :mapping, :allow_nil, :constructor, :converter)
+
+ 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
+ constructor = options[:constructor] || :new
+ converter = options[:converter] || block
+
+ ActiveSupport::Deprecation.warn('The conversion block has been deprecated, use the :converter option instead.', caller) if block_given?
+
+ reader_method(name, class_name, mapping, allow_nil, constructor)
+ writer_method(name, class_name, mapping, allow_nil, converter)
+
+ create_reflection(:composed_of, part_id, options, self)
+ end
+
+ private
+ def reader_method(name, class_name, mapping, allow_nil, constructor)
+ 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? })
+ attrs = mapping.collect {|pair| read_attribute(pair.first)}
+ object = case constructor
+ when Symbol
+ class_name.constantize.send(constructor, *attrs)
+ when Proc, Method
+ constructor.call(*attrs)
+ else
+ raise ArgumentError, 'Constructor must be a symbol denoting the constructor method to call or a Proc to be invoked.'
+ end
+ instance_variable_set("@#{name}", object)
+ end
+ instance_variable_get("@#{name}")
+ end
+ end
+
+ end
+
+ def writer_method(name, class_name, mapping, allow_nil, converter)
+ module_eval do
+ define_method("#{name}=") do |part|
+ if part.nil? && allow_nil
+ mapping.each { |pair| self[pair.first] = nil }
+ instance_variable_set("@#{name}", nil)
+ else
+ unless part.is_a?(class_name.constantize) || converter.nil?
+ part = case converter
+ when Symbol
+ class_name.constantize.send(converter, part)
+ when Proc, Method
+ converter.call(part)
+ else
+ raise ArgumentError, 'Converter must be a symbol denoting the converter method to call or a Proc to be invoked.'
+ end
+ end
+ mapping.each { |pair| self[pair.first] = part.send(pair.last) }
+ instance_variable_set("@#{name}", part.freeze)
+ end
+ end
+ end
+ end
+ end
+ end
+end
projects / depot.git / blobdiff