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<td><strong>Module</strong></td>
<td class="class-name-in-header">ActiveRecord::Aggregations::ClassMethods</td>
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<tr class="top-aligned-row">
<td><strong>In:</strong></td>
<td>
<a href="../../../files/lib/active_record/aggregations_rb.html">
lib/active_record/aggregations.rb
</a>
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<p>
Active Record implements aggregation through a macro-like class method
called <tt><a href="ClassMethods.html#M000391">composed_of</a></tt> 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:
</p>
<pre>
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
</pre>
<p>
The customer class now has the following methods to manipulate the value
objects:
</p>
<ul>
<li><tt>Customer#balance, Customer#balance=(money)</tt>
</li>
<li><tt>Customer#address, Customer#address=(address)</tt>
</li>
</ul>
<p>
These methods will operate with value objects like the ones described
below:
</p>
<pre>
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
</pre>
<p>
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 <tt>balance</tt> attribute. You
interact with the value objects just like you would any other attribute,
though:
</p>
<pre>
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
</pre>
<p>
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:
</p>
<pre>
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"
</pre>
<h2>Writing value objects</h2>
<p>
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 <a
href="../Base.html">ActiveRecord::Base</a> classes are entity objects.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
Read more about value objects on <a
href="http://c2.com/cgi/wiki?ValueObject">c2.com/cgi/wiki?ValueObject</a>
and on the dangers of not keeping value objects immutable on <a
href="http://c2.com/cgi/wiki?ValueObjectsShouldBeImmutable">c2.com/cgi/wiki?ValueObjectsShouldBeImmutable</a>
</p>
<h2>Custom constructors and converters</h2>
<p>
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 <tt><a
href="ClassMethods.html#M000391">composed_of</a></tt> allows a custom
constructor to be specified.
</p>
<p>
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.
</p>
<p>
For example, the NetworkResource model has <tt>network_address</tt> and
<tt>cidr_range</tt> attributes that should be aggregated using the
NetAddr::CIDR value class (<a
href="http://netaddr.rubyforge.org">netaddr.rubyforge.org</a>). The
constructor for the value class is called <tt>create</tt> 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:
</p>
<pre>
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
</pre>
<h2>Finding records by a value object</h2>
<p>
Once a <tt><a href="ClassMethods.html#M000391">composed_of</a></tt>
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
<tt>balance_amount</tt> equal to 20 and <tt>balance_currency</tt> equal to
"USD":
</p>
<pre>
Customer.find(:all, :conditions => {:balance => Money.new(20, "USD")})
</pre>
</div>
</div>
<div id="method-list">
<h3 class="section-bar">Methods</h3>
<div class="name-list">
<a href="#M000391">composed_of</a>
</div>
</div>
</div>
<!-- if includes -->
<div id="section">
<!-- if method_list -->
<div id="methods">
<h3 class="section-bar">Public Instance methods</h3>
<div id="method-M000391" class="method-detail">
<a name="M000391"></a>
<div class="method-heading">
<a href="ClassMethods.src/M000391.html" target="Code" class="method-signature"
onclick="popupCode('ClassMethods.src/M000391.html');return false;">
<span class="method-name">composed_of</span><span class="method-args">(part_id, options = {}, &block)</span>
</a>
</div>
<div class="method-description">
<p>
Adds reader and writer methods for manipulating a value object: <tt><a
href="ClassMethods.html#M000391">composed_of</a> :address</tt> adds
<tt>address</tt> and <tt>address=(new_address)</tt> methods.
</p>
<p>
Options are:
</p>
<ul>
<li><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><a
href="ClassMethods.html#M000391">composed_of</a> :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.
</li>
<li><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.
</li>
<li><tt>:allow_nil</tt> - Specifies that the value object will not be
instantiated when all mapped attributes are <tt>nil</tt>. Setting the value
object to <tt>nil</tt> has the effect of writing <tt>nil</tt> to all mapped
attributes. This defaults to <tt>false</tt>.
</li>
<li><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>.
</li>
<li><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>.
</li>
</ul>
<p>
Option examples:
</p>
<pre>
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) }
</pre>
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