diff options
Diffstat (limited to 'activerecord/lib/active_record/aggregations.rb')
| -rw-r--r-- | activerecord/lib/active_record/aggregations.rb | 115 |
1 files changed, 64 insertions, 51 deletions
diff --git a/activerecord/lib/active_record/aggregations.rb b/activerecord/lib/active_record/aggregations.rb index c45400d3d9..83a9ab46c5 100644 --- a/activerecord/lib/active_record/aggregations.rb +++ b/activerecord/lib/active_record/aggregations.rb @@ -9,11 +9,13 @@ module ActiveRecord 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: + # 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) @@ -68,9 +70,10 @@ module ActiveRecord # 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: + # 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 @@ -79,8 +82,8 @@ module ActiveRecord # 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: + # 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" @@ -91,38 +94,43 @@ module ActiveRecord # # == 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. + # 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. + # 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. + # 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 + # 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. + # 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. + # 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: + # 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, @@ -149,9 +157,9 @@ module ActiveRecord # # == 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": + # 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")}) # @@ -160,23 +168,28 @@ module ActiveRecord # <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>: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. + # 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. + # * <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>. + # * <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) |