module ActiveRecord
  # See ActiveRecord::Aggregations::ClassMethods for documentation
  module Aggregations
    extend ActiveSupport::Concern

    def initialize_dup(*) # :nodoc:
      @aggregation_cache = {}
      super
    end

    def reload(*) # :nodoc:
      clear_aggregation_cache
      super
    end

    private

      def clear_aggregation_cache # :nodoc:
        @aggregation_cache.clear if persisted?
      end

      def init_internals # :nodoc:
        @aggregation_cache = {}
        super
      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(amount currency)
      #     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 = 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
      # <tt>Money#exchange_to</tt> 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 +RuntimeError+.
      #
      # 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://www.rubydoc.info/gems/netaddr/1.5.0/NetAddr/CIDR).
      # 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"))
      #
      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)

          reflection = ActiveRecord::Reflection.create(:composed_of, part_id, nil, options, self)
          Reflection.add_aggregate_reflection self, part_id, reflection
        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? { |key, _| !_read_attribute(key).nil? })
                attrs = mapping.collect { |key, _| _read_attribute(key) }
                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

              hash_from_multiparameter_assignment = part.is_a?(Hash) &&
                part.each_key.all? { |k| k.is_a?(Integer) }
              if hash_from_multiparameter_assignment
                raise ArgumentError unless part.size == part.each_key.max
                part = klass.new(*part.sort.map(&:last))
              end

              if part.nil? && allow_nil
                mapping.each { |key, _| self[key] = nil }
                @aggregation_cache[name] = nil
              else
                mapping.each { |key, value| self[key] = part.send(value) }
                @aggregation_cache[name] = part.freeze
              end
            end
          end
      end
  end
end