Module | ActiveRecord::Aggregations::ClassMethods
In: | lib/active_record/aggregations.rb

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:

      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:

- `Customer#balance, Customer#balance=(money)`
- `Customer#address, Customer#address=(address)`

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 <=> 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 any other attribute, though:

      customer.balance = Money.new(20)     # sets the Money value object and the attribute
      customer.balance                     # => Money value object
      customer.balance.exchanged_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. Example:

      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 `==` and `<=>` 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#exchanged_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.

Read more about value objects on c2.com/cgi/wiki?ValueObject and on the dangers of not keeping value objects immutable on c2.com/cgi/wiki?ValueObjectsShouldBeImmutable

## 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.find(:all, :conditions => {:balance => Money.new(20, "USD")})

### Methods

composed_of

### Public Instance methods

composed_of(part_id, options = {}, &block)

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

Options are:

- `:class_name` - specify 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 a number of mapping arrays (attribute, parameter) that bind an attribute name to a constructor parameter on the value class.
- `:allow_nil` - specifies that the aggregate object will not be instantiated when all mapped attributes are `nil`. Setting the aggregate class to `nil` has the effect of writing `nil` to all mapped attributes. This defaults to `false`.

An optional block can be passed to convert the argument that is passed to the writer method into an instance of `:class_name`. The block will only be called if the argument is not already an instance of `:class_name`.

Option examples:

      composed_of :temperature, :mapping => %w(reading celsius)
      composed_of(:balance, :class_name => "Money", :mapping => %w(balance amount)) {|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

[Validate]