Finding Records by a Value Object in Java

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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":
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Customerwhere(:balance => Moneynew(20, "USD"))
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The Money Gem A common approach to using composed_of is in conjunction with the money gem12
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class Expense < ActiveRecord::Base composed_of :cost, :class_name => "Money", :mapping => [%w(cents cents), %w(currency currency_as_string)], :constructor => Procnew do |cents, currency| Moneynew(cents || 0, currency || Moneydefault_currency) end end
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Remember to add a migration with the 2 columns, the integer cents and the string currency that money needs
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12 http://githubcom/FooBarWidget/money/
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class CreateExpenses < ActiveRecord::Migration def selfup create_table :expenses do |table| tableinteger :cents tablestring :currency end end def selfdown drop_table :expenses end end
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Now when asking for or setting the cost of an item would use a Money instance
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>> >> >> => >> => expense = Expensecreate(:cost => Moneynew(1000, "USD")) cost = expensecost costcents 1000 expensecurrency "USD"
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Active Record
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In this section, we ll talk about one strategy for breaking out functionality that is shared between disparate model classes Instead of using inheritance, we ll put the shared code into modules In the section Polymorphic has_many Relationships, we described how to add a commenting feature to our recurring sample Time and Expenses application We ll continue fleshing out that example, since it lends itself to factoring out into modules The requirements we ll implement are as follows: Both users and approvers should be able to add their comments to a Timesheet or ExpenseReport Also, since comments are indicators that a timesheet or expense report requires extra scrutiny or processing time, administrators of the application should be able to easily view a list of recent comments Human nature being what it is, administrators occasionally gloss over the comments without actually reading them, so the requirements specify that a mechanism should be provided for marking comments as OK first by the approver, then by the administrator Again, here is the polymorphic has_many :comments, :as => :commentable that we used as the foundation for this functionality:
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class Timesheet < ActiveRecord::Base has_many :comments, :as => :commentable end
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class ExpenseReport < ActiveRecord::Base has_many :comments, :as => :commentable end
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9: Advanced Active Record
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class Comment < ActiveRecord::Base belongs_to :commentable, :polymorphic => true end
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Next we enable the controller and action for the administrator that list the 10 most recent comments with links to the item to which they are attached
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class Comment < ActiveRecord::Base scope :recent, order('created_at desc')limit(10) end class CommentsController < ApplicationController before_filter :require_admin, :only => :recent expose(:recent_comments) { Commentrecent } end
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Here s some of the simple view template used to display the recent comments
%ulrecentcomments - recent_commentseach do |comment| %licomment %h4= commentcreated_at = commenttext meta Comment on: = link_to commentcommentabletitle, commentcommentable Yes, this would result in N+1 selects
So far, so good The polymorphic association makes it easy to access all types of comments in one listing In order to find all of the unreviewed comments for an item, we can use a named scope on the Comment class together with the comments association
class Comment < ActiveRecord::Base scope :unreviewed, where(:reviewed => false) end >> timesheetcommentsunreviewed
Both Timesheet and ExpenseReport currently have identical has_many methods for comments Essentially, they both share a common interface They re commentable! To minimize duplication, we could specify common interfaces that share code in Ruby by including a module in each of those classes, where the module contains the code common to all implementations of the common interface So, mostly for the sake of
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example, let s go ahead and define a Commentable module to do just that, and include it in our model classes:
module Commentable has_many :comments, :as => :commentable end class Timesheet < ActiveRecord::Base include Commentable end class ExpenseReport < ActiveRecord::Base include Commentable end
Whoops, this code doesn t work! To fix it, we need to understand an essential aspect of the way that Ruby interprets our code dealing with open classes
9101 A Review of Class Scope and Contexts
In many other interpreted OO programming languages, you have two phases of execution one in which the interpreter loads the class definitions and says this is the definition of what I have to work with, followed by the phase in which it executes the code This makes it difficult (though not necessarily impossible) to add new methods to a class dynamically during execution In contrast, Ruby lets you add methods to a class at any time In Ruby, when you type class MyClass, you re doing more than simply telling the interpreter to define a class; you re telling it to execute the following code in the scope of this class Let s say you have the following Ruby script:
1 2 3 4 5 6 class Foo < ActiveRecord::Base has_many :bars end class Foo < ActiveRecord::Base belongs_to :spam end