Home > Articles > Web Development

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

9.4 Single-Table Inheritance (STI)

A lot of applications start out with a User model of some sort. Over time, as different kinds of users emerge, it might make sense to make a greater distinction between them. Admin and Guest classes are introduced as subclasses of User. Now the shared behavior can reside in User, and the subtype behavior can be pushed down to subclasses. However, all user data can still reside in the users table—all you need to do is introduce a type column that will hold the name of the class to be instantiated for a given row.

To continue explaining single-table inheritance, let’s turn back to our example of a recurring Timesheet class. We need to know how many billable_hours are outstanding for a given user. The calculation can be implemented in various ways, but in this case we’ve chosen to write a pair of class and instance methods on the Timesheet class:


 1 class Timesheet < ActiveRecord::Base
 2   ...
 3
 4   def billable_hours_outstanding
 5     if submitted?
 6       billable_weeks.map(&:total_hours).sum
 7     else
 8       0
 9     end
10   end
11
12   def self.billable_hours_outstanding_for(user)
13     user.timesheets.map(&:billable_hours_outstanding).sum
14   end
15
16 end

I’m not suggesting that this is good code. It works, but it’s inefficient and that if/else condition is a little fishy. Its shortcomings become apparent once requirements emerge about marking a Timesheet as paid. It forces us to modify Timesheet’s billable_hours_outstanding method again:


1 def billable_hours_outstanding
2   if submitted? && not paid?
3     billable_weeks.map(&:total_hours).sum
4   else
5     0
6   end
7 end

That latest change is a clear violation of the open-closed principle,6 which urges you to write code that is open for extension but closed for modification. We know that we violated the principle, because we were forced to change the billable_hours_outstanding method to accommodate the new Timesheet status. Though it may not seem like a large problem in our simple example, consider the amount of conditional code that will end up in the Timesheet class once we start having to implement functionality such as paid_hours and unsubmitted_hours.

So what’s the answer to this messy question of the constantly changing conditional? Given that you’re reading the section of the book about single-table inheritance, it’s probably no big surprise that we think one good answer is to use object-oriented inheritance. To do so, let’s break our original Timesheet class into four classes.

 1 class Timesheet < ActiveRecord::Base
 2   # nonrelevant code ommited
 3
 4   def self.billable_hours_outstanding_for(user)
 5     user.timesheets.map(&:billable_hours_outstanding).sum
 6   end
 7 end
 8
 9 class DraftTimesheet < Timesheet
10   def billable_hours_outstanding
11     0
12   end
13 end
14
15 class SubmittedTimesheet < Timesheet
16   def billable_hours_outstanding
17     billable_weeks.map(&:total_hours).sum
18   end
19 end

Now when the requirements demand the ability to calculate partially paid timesheets, we need only add some behavior to a PaidTimesheet class. No messy conditional statements in sight!


1 class PaidTimesheet < Timesheet
2   def billable_hours_outstanding
3     billable_weeks.map(&:total_hours).sum - paid_hours
4   end
5 end

9.4.1 Mapping Inheritance to the Database

Mapping object inheritance effectively to a relational database is not one of those problems with a definitive solution. We’re only going to talk about the one mapping strategy that Rails supports natively, which is single-table inheritance, called STI for short.

In STI, you establish one table in the database to hold all the records for any object in a given inheritance hierarchy. In Active Record STI, that one table is named after the top parent class of the hierarchy. In the example we’ve been considering, that table would be named timesheets.

Hey, that’s what it was called before, right? Yes, but to enable STI, we have to add a type column to contain a string representing the type of the stored object. The following migration would properly set up the database for our example:


1 class AddTypeToTimesheet < ActiveRecord::Migration
2   def change
3     add_column :timesheets, :type, :string
4   end
5 end

No default value is needed. Once the type column is added to an Active Record model, Rails will automatically take care of keeping it populated with the right value. Using the console, we can see this behavior in action:


>> d = DraftTimesheet.create
>> d.type
=> 'DraftTimesheet'

When you try to find an object using the query methods of a base STI class, Rails will automatically instantiate objects using the appropriate subclass. This is especially useful in polymorphic situations, such as the timesheet example we’ve been describing, where we retrieve all the records for a particular user and then call methods that behave differently depending on the object’s class.


>> Timesheet.first
=> #<DraftTimesheet:0x2212354...>

9.4.2 STI Considerations

Although Rails makes it extremely simple to use single-table inheritance, there are four caveats that you should keep in mind.

First, you cannot have an attribute on two different subclasses with the same name but a different type. Since Rails uses one table to store all subclasses, these attributes with the same name occupy the same column in the table. Frankly, there’s not much of a reason that should be a problem unless you’ve made some pretty bad data-modeling decisions.

Second and more important, you need to have one column per attribute on any subclass, and any attribute that is not shared by all the subclasses must accept nil values. In the recurring example, PaidTimesheet has a paid_hours column that is not used by any of the other subclasses. DraftTimesheet and SubmittedTimesheet will not use the paid_hours column and leave it as null in the database. In order to validate data for columns not shared by all subclasses, you must use Active Record validations and not the database.

Third, it is not a good idea to have subclasses with too many unique attributes. If you do, you will have one database table with many null values in it. Normally, a tree of subclasses with a large number of unique attributes suggests that something is wrong with your application design and that you should refactor. If you have an STI table that is getting out of hand, it is time to reconsider your decision to use inheritance to solve your particular problem. Perhaps your base class is too abstract?

Finally, legacy database constraints may require a different name in the database for the type column. In this case, you can set the new column name using the class setter method inheritance_column in the base class. For the Timesheet example, we could do the following:


1 class Timesheet < ActiveRecord::Base
2   self.inheritance_column = 'object_type'
3 end

Now Rails will automatically populate the object_type column with the object’s type.

9.4.3 STI and Associations

It seems pretty common for applications, particularly data-management ones, to have models that are very similar in terms of their data payload, mostly varying in their behavior and associations to each other. If you used object-oriented languages prior to Rails, you’re probably already accustomed to breaking down problem domains into hierarchical structures.

Take, for instance, a Rails application that deals with the population of states, counties, cities, and neighborhoods. All of these are places, which might lead you to define an STI class named Place as shown in Listing 9.2. I’ve also included the database schema for clarity:7

Listing 9.2 The Places Database Schema and the Place Class


 1 # == Schema Information
 2 #
 3 # Table name: places
 4 #
 5 #  id  :integer(11)  not null, primary key
 6 #  region_id  :integer(11)
 7 #  type  :string(255)
 8 #  name  :string(255)
 9 #  description  :string(255)
10 #  latitude  :decimal(20, 1)
11 #  longitude  :decimal(20, 1)
12 #  population  :integer(11)
13 #  created_at  :datetime
14 #  updated_at  :datetime
15
16 class Place < ActiveRecord::Base
17 end

Place is in essence an abstract class. It should not be instantiated, but there is no foolproof way to enforce that in Ruby. (No big deal, this isn’t Java!) Now let’s go ahead and define concrete subclasses of Place:


 1 class State < Place
 2   has_many :counties, foreign_key: 'region_id'
 3 end
 4
 5 class County < Place
 6   belongs_to :state, foreign_key: 'region_id'
 7   has_many :cities, foreign_key: 'region_id'
 8 end
 9
10 class City < Place
11   belongs_to :county, foreign_key: 'region_id'
12 end

You might be tempted to try adding a cities association to State, knowing that has_many :through works with both belongs_to and has_many target associations. It would make the State class look something like this:


1 class State < Place
2   has_many :counties, foreign_key: 'region_id'
3   has_many :cities, through: :counties
4 end

That would certainly be cool if it worked. Unfortunately, in this particular case, since there’s only one underlying table that we’re querying, there simply isn’t a way to distinguish among the different kinds of objects in the query:

Mysql::Error: Not unique table/alias: 'places': SELECT places.* FROM
places INNER JOIN places ON places.region_id = places.id WHERE
((places.region_id = 187912) AND ((places.type = 'County'))) AND
((places.`type` = 'City' ))

What would we have to do to make it work? Well, the most realistic would be to use specific foreign keys instead of trying to overload the meaning of region_id for all the subclasses. For starters, the places table would look like the example in Listing 9.3.

Listing 9.3 The Places Database Schema Revised

# == Schema Information
#
# Table name: places
#
#  id  :integer(11)  not null, primary key
#  state_id  :integer(11)
#  county_id  :integer(11)
#  type  :string(255)
#  name  :string(255)
#  description  :string(255)
#  latitude  :decimal(20, 1)
#  longitude  :decimal(20, 1)
#  population  :integer(11)
#  created_at  :datetime
#  updated_at  :datetime

The subclasses would be simpler without the :foreign_key options on the associations. Plus you could use a regular has_many relationship from State to City instead of the more complicated has_many :through.


 1 class State < Place
 2   has_many :counties
 3   has_many :cities
 4 end
 5
 6 class County < Place
 7   belongs_to :state
 8   has_many :cities
 9 end
10
11 class City < Place
12   belongs_to :county
13 end

Of course, all those null columns in the places table won’t win you any friends with relational database purists. That’s nothing, though. Just a little bit later in this chapter, we’ll take a second, more in-depth look at polymorphic has_many relationships, that will make the purists positively hate you.

  • + Share This
  • 🔖 Save To Your Account

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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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