diff options
Diffstat (limited to 'activerecord/lib/active_record/aggregations.rb')
| -rw-r--r-- | activerecord/lib/active_record/aggregations.rb | 310 |
1 files changed, 155 insertions, 155 deletions
diff --git a/activerecord/lib/active_record/aggregations.rb b/activerecord/lib/active_record/aggregations.rb index 8979b13286..5ca8fe576e 100644 --- a/activerecord/lib/active_record/aggregations.rb +++ b/activerecord/lib/active_record/aggregations.rb @@ -24,161 +24,161 @@ module ActiveRecord 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")) - # + # 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. |