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Diffstat (limited to 'activerecord/lib/active_record/aggregations.rb')
| -rw-r--r-- | activerecord/lib/active_record/aggregations.rb | 261 |
1 files changed, 261 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/aggregations.rb b/activerecord/lib/active_record/aggregations.rb new file mode 100644 index 0000000000..3db8e0716b --- /dev/null +++ b/activerecord/lib/active_record/aggregations.rb @@ -0,0 +1,261 @@ +module ActiveRecord + # = Active Record Aggregations + module Aggregations # :nodoc: + extend ActiveSupport::Concern + + def clear_aggregation_cache #:nodoc: + @aggregation_cache.clear if persisted? + 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). + # + # 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 <=> other_money.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 with any other attribute: + # + # 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. + # + # customer.address_street = "Hyancintvej" + # customer.address_city = "Copenhagen" + # customer.address # => Address.new("Hyancintvej", "Copenhagen") + # + # customer.address_street = "Vesterbrogade" + # customer.address # => Address.new("Hyancintvej", "Copenhagen") + # customer.clear_aggregation_cache + # customer.address # => Address.new("Vesterbrogade", "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. The + # Money#exchange_to method is an example of this. It 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.where(:balance => Money.new(20, "USD")).all + # + 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 of the attribute in the value object. The + # order in which mappings are defined determines 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>. If <tt>:allow_nil</tt> is set to true, the converter + # can return nil to skip the assignment. + # + # 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 = {}) + 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] + + reader_method(name, class_name, mapping, allow_nil, constructor) + writer_method(name, class_name, mapping, allow_nil, converter) + + create_reflection(:composed_of, part_id, nil, options, self) + end + + private + def reader_method(name, class_name, mapping, allow_nil, constructor) + define_method(name) do + if @aggregation_cache[name].nil? && (!allow_nil || mapping.any? {|pair| !read_attribute(pair.first).nil? }) + attrs = mapping.collect {|pair| read_attribute(pair.first)} + object = constructor.respond_to?(:call) ? + constructor.call(*attrs) : + class_name.constantize.send(constructor, *attrs) + @aggregation_cache[name] = object + end + @aggregation_cache[name] + end + end + + def writer_method(name, class_name, mapping, allow_nil, converter) + define_method("#{name}=") do |part| + klass = class_name.constantize + unless part.is_a?(klass) || converter.nil? || part.nil? + part = converter.respond_to?(:call) ? converter.call(part) : klass.send(converter, part) + end + + if part.nil? && allow_nil + mapping.each { |pair| self[pair.first] = nil } + @aggregation_cache[name] = nil + else + mapping.each { |pair| self[pair.first] = part.send(pair.last) } + @aggregation_cache[name] = part.freeze + end + end + end + end + end +end |