Home > Articles > Web Development

This chapter is from the book

This chapter is from the book

9.9 Modules for Reusing Common Behavior

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” in this chapter, 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 and then by the administrator.

Again, here is the polymorphic has_many :comments, as: :commentable that we used as the foundation for this functionality:

 1 class Timesheet < ActiveRecord::Base
 2   has_many :comments, as: :commentable
 3 end
 5 class ExpenseReport < ActiveRecord::Base
 6   has_many :comments, as: :commentable
 7 end
 9 class Comment < ActiveRecord::Base
10   belongs_to :commentable, polymorphic: true
11 end

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.

1 class Comment < ActiveRecord::Base
2   scope :recent, -> { order('created_at desc').limit(10) }
3 end
5 class CommentsController < ApplicationController
6   before_action :require_admin, only: :recent
7   expose(:recent_comments) { Comment.recent }
8 end

Here’s some of the simple view template used to display the recent comments:

1 %ul.recent.comments
2   - recent_comments.each do |comment|
3     %li.comment
4       %h4= comment.created_at
5       = comment.text
6       .meta
7         Comment on:
8         = link_to comment.commentable.title, comment.commentable
9         # 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 the unreviewed comments for an item, we can use a named scope on the Comment class together with the comments association.

1 class Comment < ActiveRecord::Base
2   scope :unreviewed, -> { where(reviewed: false) }
3 end
5 >> timesheet.comments.unreviewed

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 example, let’s go ahead and define a Commentable module to do just that and include it in our model classes:

 1 module Commentable
 2   has_many :comments, as: :commentable
 3 end
 5 class Timesheet < ActiveRecord::Base
 6   include Commentable
 7 end
 9 class ExpenseReport < ActiveRecord::Base
10   include Commentable
11 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.

9.9.1 A Review of Class Scope and Contexts

In many other interpreted, object-oriented 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,” and the second 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 class Foo < ActiveRecord::Base
2   has_many :bars
3 end
4 class Foo < ActiveRecord::Base
5   belongs_to :spam
6 end

When the interpreter gets to line 1, we are telling it to execute the following code (up to the matching end) in the context of the Foo class object. Because the Foo class object doesn’t exist yet, it goes ahead and creates the class. At line 2, we execute the statement has_many :bars in the context of the Foo class object. Whatever the has_many method does, it does right now.

When we again say class Foo at line 4, we are once again telling the interpreter to execute the following code in the context of the Foo class object, but this time, the interpreter already knows about class Foo; it doesn’t actually create another class. Therefore, on line 5, we are simply telling the interpreter to execute the belongs_to :spam statement in the context of that same Foo class object.

In order to execute the has_many and belongs_to statements, those methods need to exist in the context in which they are executed. Because these are defined as class methods in ActiveRecord::Base, and we have previously defined class Foo as extending ActiveRecord::Base, the code will execute without a problem.

However, let’s say we defined our Commentable module like this:

1 module Commentable
2   has_many :comments, as: :commentable
3 end

In this case, we get an error when it tries to execute the has_many statement. That’s because the has_many method is not defined in the context of the Commentable module object.

Given what we now know about how Ruby is interpreting the code, we now realize that what we really want is for that has_many statement to be executed in the context of the including class.

9.9.2 The included Callback

Luckily, Ruby’s Module class defines a handy callback that we can use to do just that. If a Module object defines the method included, it gets run whenever that module is included in another module or class. The argument passed to this method is the module/class object into which this module is being included.

We can define an included method on our Commentable module object so that it executes the has_many statement in the context of the including class (Timesheet, ExpenseReport, etc.):

1 module Commentable
2   def self.included(base)
3     base.class_eval do
4       has_many :comments, as: :commentable
5     end
6   end
7 end

Now when we include the Commentable module in our model classes, it will execute the has_many statement just as if we had typed it into each of those classes’ bodies.

The technique is common enough, within Rails and gems, that it was added as a first-class concept in the Active Support API as of Rails 3. The previous example becomes shorter and easier to read as a result:

1  # app/models/concerns/commentable.rb
2 module Commentable
3   extend ActiveSupport::Concern
4   included do
5     has_many :comments, as: :commentable
6   end
7 end

Whatever is inside of the included block will get executed in the class context of the class where the module is included.

As of version 4.0, Rails includes the directory app/models/concerns as a place to keep all your application’s model concerns. Any file found within this directory will automatically be part of the application load path.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020