9: Advanced Active Record in Java

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9: Advanced Active Record
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ActiveRecord::Base Instead you make them part of a parent model using the composed_of class method This is a form of composition, called an Aggregate in DDD
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The attributes of the Value Object are stored in the database together with the parent object and composed_of provides a means to interact with those values as a single object A simple example is of a Person with a single Address To model this using composition, first we need a Person model with fields for the Address Create it with the following migration:
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class CreatePeople < ActiveRecord::Migration def selfup create_table :people do |t| tstring :name tstring :address_city tstring :address_state end end end
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The Person model looks like this:
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class Person < ActiveRecord::Base composed_of :address, :mapping => [%w(address_city city), %w(address_state state)] end
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We d need a corresponding Address object which looks like this:
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class Address attr_reader :city, :state def initialize(city, state) @city, @state = city, state end def ==(other_address) city == other_addresscity && state == other_addressstate end end
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Note that this is just a standard Ruby object that does not inherit from ActiveRecord::Base We have defined reader methods for our attributes and are assigning them upon initialization We also have to define our own == method for use in comparisons Wrapping this all up we get the following usage:
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>> >> >> >> => gary = Personcreate(:name => "Gary") garyaddress_city = "Brooklyn" garyaddress_state = "NY" garyaddress #<Address:0x20bc118 @state="NY", @city="Brooklyn">
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Alternately you can instantiate the address directly and assign it using the address accessor:
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>> garyaddress = Addressnew("Brooklyn", "NY") >> garyaddress => #<Address:0x20bc118 @state="NY", @city="Brooklyn">
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It s also important to treat value objects as immutable Don t allow them to be changed after creation Instead, create a new object instance with the new value instead Active Record will not 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
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Active Record
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By default value objects are initialized by calling the new constructor of the value class with each of the mapped attributes, in the order specified by the :mapping option, as arguments If for some reason your value class does not work well with that convention, composed_of allows a custom constructor to be specified When a new value object is assigned to its parent, 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 (when needed) For example, consider the NetworkResource model with network_address and cidr_range attributes that should be contained in a NetAddr::CIDR value class11 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 are used to meet the requirements:
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class NetworkResource < ActiveRecord::Base composed_of :cidr, :class_name => 'NetAddr::CIDR', :mapping => [ %w(network_address network), %w(cidr_range bits) ], :allow_nil => true,
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11 Actual objects from the NetAddr gem available at http://netaddrrubyforgeorg
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9: Advanced Active Record
:constructor => Procnew { |network_address, cidr_range| NetAddr::CIDRcreate("#{network_address}/#{cidr_range}") }, :converter => Procnew { |value| NetAddr::CIDRcreate(valueis_a (Array) valuejoin('/') : value) } end # This calls the :constructor network_resource = NetworkResourcenew(:network_address => '19216801', :cidr_range => 24) # These assignments will both use the :converter network_resourcecidr = [ '19216821', 8 ] network_resourcecidr = '19216801/24' # This assignment won't use the :converter as the value is already an instance of the value class network_resourcecidr = NetAddr::CIDRcreate('19216821/8') # Saving and then reloading will use the :constructor on reload network_resourcesave network_resourcereload