Module: ActiveRecord::Aggregations::ClassMethods
- Included in:
- Base
- Defined in:
- activerecord/lib/active_record/aggregations.rb
Overview
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)
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 <=> 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 ==
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. The Money#exchange_to
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 c2.com/cgi/wiki?ValueObject and on the dangers of not keeping value objects immutable on c2.com/cgi/wiki?ValueObjectsShouldBeImmutable
Custom constructors and converters
By default value objects are initialized by calling the new
constructor of the value class passing each of the mapped attributes, in the order specified by the :mapping
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 (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 :constructor
and :converter
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 address_street
equal to “May Street” and address_city
equal to “Chicago”:
Customer.where(address: Address.new("May Street", "Chicago"))
Instance Method Summary collapse
-
#composed_of(part_id, options = {}) ⇒ Object
Adds reader and writer methods for manipulating a value object:
composed_of :address
addsaddress
andaddress=(new_address)
methods.
Instance Method Details
#composed_of(part_id, options = {}) ⇒ Object
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. Socomposed_of :address
will by default be linked to the Address class, but if the real class name isCompanyAddress
, 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 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. -
:allow_nil
- Specifies that the value object will not be instantiated when all mapped attributes arenil
. Setting the value object tonil
has the effect of writingnil
to all mapped attributes. This defaults tofalse
. -
: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
. If:allow_nil
is set to true, the converter can returnnil
to skip the assignment.
Option examples:
composed_of :temperature, mapping: %w(reading celsius)
composed_of :balance, class_name: "Money", mapping: %w(balance amount)
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) }
222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 |
# File 'activerecord/lib/active_record/aggregations.rb', line 222 def composed_of(part_id, = {}) .assert_valid_keys(:class_name, :mapping, :allow_nil, :constructor, :converter) unless self < Aggregations include Aggregations end name = part_id.id2name class_name = [:class_name] || name.camelize mapping = [:mapping] || [ name, name ] mapping = [ mapping ] unless mapping.first.is_a?(Array) allow_nil = [:allow_nil] || false constructor = [:constructor] || :new converter = [: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, , self) Reflection.add_aggregate_reflection self, part_id, reflection end |