composed_of(part_id, options = {}) public

Adds reader and writer methods for manipulating a value object: composed_of :address adds address and address=(new_address) methods.

Options are:

  • :class_name - Specifies the class name of the association. Use it only if that name can’t be inferred from the part id. So composed_of :address 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.
  • :mapping - 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.
  • :allow_nil - 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.
  • :constructor - 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 :mapping option, as arguments and uses them to instantiate a :class_name object. The default is :new.
  • :converter - A symbol specifying the name of a class method of :class_name 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 :class_name.

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) }
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September 26, 2008
4 thanks

Example of composed_of composition class implementation

If we have following code in model:

composed_of :temperature, :mapping => %w(celsius)

Then our composition class can be this:

class Temperature
  def initialize(celsius)
    @celsius = celsius

  # This method is called by ActiveRecord, when record is saved.
  # Result of this method will be stored in table in "celsius" field,
  # and later when the record is loaded again, this will go to 
  # our Temperature#new constructor.
  def celsius

  # This is example of method that we can add to make this composition useful.
  def farenheit 
    @celsius * 9/5 + 32
July 15, 2008
1 thank

Mapping Order

Something I always forget is the order in which each mapping should be specified.

The first item is the attribute name in the ActiveRecord model, and the second is the name of the attribute in the ValueObject (the writer uses it to read from the VO).

Furthermore, the order in which mapping pairs are specified should be the same as the order the attributes are specified in the ValueObject’s initialize method (for the reader to be able to instantiate a VO with the record’s values).