When teams use Code Climate to improve the quality of their Rails applications, they learn to break the habit of allowing models to get fat. “Fat models” cause maintenance issues in large apps. Only incrementally better than cluttering controllers with domain logic, they usually represent a failure to apply the Single Responsibility Principle (SRP). “Anything related to what a user does” is not a single responsibility.
Early on, SRP is easier to apply. ActiveRecord classes handle persistence, associations and not much else. But bit-by-bit, they grow. Objects that are inherently responsible for persistence become the de facto owner of all business logic as well. And a year or two later you have a
User class with over 500 lines of code, and hundreds of methods in it’s public interface. Callback hell ensues.
As you add more intrinsic complexity (read: features!) to your application, the goal is to spread it across a coordinated set of small, encapsulated objects (and, at a higher level, modules) just as you might spread cake batter across the bottom of a pan. Fat models are like the big clumps you get when you first pour the batter in. Refactor to break them down and spread out the logic evenly. Repeat this process and you’ll end up with a set of simple objects with well defined interfaces working together in a veritable symphony.
You may be thinking:
“But Rails makes it really hard to do OOP right!”
I used to believe that too. But after some exploration and practice, I realized that Rails (the framework) doesn’t impede OOP all that much. It’s the Rails “conventions” that don’t scale, or, more specifically, the lack of conventions for managing complexity beyond what the Active Record pattern can elegantly handle. Fortunately, we can apply OO-based principles and best practices where Rails is lacking.
Don’t Extract Mixins from Fat Models
Let’s get this out of the way. I discourage pulling sets of methods out of a large ActiveRecord class into “concerns”, or modules that are then mixed in to only one model. I once heard someone say:
“Any application with an
app/concernsdirectory is concerning.”
And I agree. Prefer composition to inheritance. Using mixins like this is akin to “cleaning” a messy room by dumping the clutter into six separate junk drawers and slamming them shut. Sure, it looks cleaner at the surface, but the junk drawers actually make it harder to identify and implement the decompositions and extractions necessary to clarify the domain model.
Now on to the refactorings!
1. Extract Value Objects
Value Objects are simple objects whose equality is dependent on their value rather than an identity. They are usually immutable.
Pathname are examples from Ruby’s standard library, but your application can (and almost certainly should) define domain-specific Value Objects as well. Extracting them from ActiveRecords is low hanging refactoring fruit.
In Rails, Value Objects are great when you have an attribute or small group of attributes that have logic associated with them. Anything more than basic text fields and counters are candidates for Value Object extraction.
For example, a text messaging application I worked on had a
PhoneNumber Value Object. An e-commerce application needs a
Money class. Code Climate has a Value Object named
Rating that represents a simple A - F grade that each class or module receives. I could (and originally did) use an instance of a Ruby
Rating allows me to combine behavior with the data:
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ConstantSnapshot then exposes an instance of
Rating in its public interface:
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Beyond slimming down the ConstantSnapshot class, this has a number of advantages:
#better_than?methods provide a more expressive way to compare ratings than Ruby’s built-in operators (e.g.
#eql?makes it possible to use a
Ratingas a hash key. Code Climate uses this to idiomatically group constants by their ratings using
#to_smethod allows me to interpolate a
Ratinginto a string (or template) without any extra work.
- The class definition provides a convenient place for a factory method, returning the correct
Ratingfor a given “remediation cost” (the estimated time it would take to fix all of the smells in a given class).
2. Extract Service Objects
Some actions in a system warrant a Service Object to encapsulate their operation. I reach for Service Objects when an action meets one or more of these criteria:
- The action is complex (e.g. closing the books at the end of an accounting period)
- The action reaches across multiple models (e.g. an e-commerce purchase using
- The action interacts with an external service (e.g. posting to social networks)
- The action is not a core concern of the underlying model (e.g. sweeping up outdated data after a certain time period).
- There are multiple ways of performing the action (e.g. authenticating with an access token or password). This is the Gang of Four Strategy pattern.
As an example, we could pull a
User#authenticate method out into a
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SessionsController would look like this:
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3. Extract Form Objects
When multiple ActiveRecord models might be updated by a single form submission, a Form Object can encapsulate the aggregation. This is far cleaner than using
accepts_nested_attributes_for, which, in my humble opinion, should be deprecated. A common example would be a signup form that results in the creation of both a
Company and a
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I’ve started using Virtus in these objects to get ActiveRecord-like attribute functionality. The Form Object quacks like an ActiveRecord, so the controller remains familiar:
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This works well for simple cases like the above, but if the persistence logic in the form gets too complex you can combine this approach with a Service Object. As a bonus, since validation logic is often contextual, it can be defined in the place exactly where it matters instead of needing to guard validations in the ActiveRecord itself.
4. Extract Query Objects
For complex SQL queries littering the definition of your ActiveRecord subclass (either as scopes or class methods), consider extracting query objects. Each query object is responsible for returning a result set based on business rules. For example, a Query Object to find abandoned trials might look like this:
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You might use it in a background job to send emails:
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ActiveRecord::Relation instances are first class citizens as of Rails 3, they make a great input to a Query Object. This allows you to combine queries using composition:
Don’t bother testing a class like this in isolation. Use tests that exercise the object and the database together to ensure the correct rows are returned in the right order and any joins or eager loads are performed (e.g. to avoid N + 1 query bugs).
5. Introduce View Objects
If logic is needed purely for display purposes, it does not belong in the model. Ask yourself, “If I was implementing an alternative interface to this application, like a voice-activated UI, would I need this?”. If not, consider putting it in a helper or (often better) a View object.
For example, the donut charts in Code Climate break down class ratings based on a snapshot of the codebase (e.g. Rails on Code Climate) and are encapsulated as a View:
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I often find a one-to-one relationship between Views and ERB (or Haml/Slim) templates. This has led me to start investigating implementations of the Two Step View pattern that can be used with Rails, but I don’t have a clear solution yet.
Note: The term “Presenter” has caught on in the Ruby community, but I avoid it for its baggage and conflicting use. The “Presenter” term was introduced by Jay Fields to describe what I refer to above as “Form Objects”. Also, Rails unfortunately uses the term “view” to describe what are otherwise known as “templates”. To avoid ambiguity, I sometimes refer to these View objects as “View Models”.
6. Extract Policy Objects
Sometimes complex read operations might deserve their own objects. In these cases I reach for a Policy Object. This allows you to keep tangential logic, like which users are considered active for analytics purposes, out of your core domain objects. For example:
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This Policy Object encapsulates one business rule, that a user is considered active if they have a confirmed email address and have logged in within the last two weeks. You can also use Policy Objects for a group of business rules like an
Authorizer that regulates which data a user can access.
Policy Objects are similar to Service Objects, but I use the term “Service Object” for write operations and “Policy Object” for reads. They are also similar to Query Objects, but Query Objects focus on executing SQL to return a result set, whereas Policy Objects operate on domain models already loaded into memory.
7. Extract Decorators
Decorators let you layer on functionality to existing operations, and therefore serve a similar purpose to callbacks. For cases where callback logic only needs to run in some circumstances or including it in the model would give the model too many responsibilities, a Decorator is useful.
Posting a comment on a blog post might trigger a post to someone’s Facebook wall, but that doesn’t mean the logic should be hard wired into the
Comment class. One sign you’ve added too many responsibilities in callbacks is slow and brittle tests or an urge to stub out side effects in wholly unrelated test cases.
Here’s how you might extract Facebook posting logic into a Decorator:
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And how a controller might use it:
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Decorators differ from Service Objects because they layer on responsibilities to existing interfaces. Once decorated, collaborators just treat the
FacebookCommentNotifier instance as if it were a
Comment. In its standard library, Ruby provides a number of facilities to make building decorators easier with metaprogramming.
Even in a Rails application, there are many tools to manage complexity in the model layer. None of them require you to throw out Rails. ActiveRecord is a fantastic library, but any pattern breaks down if you depend on it exclusively. Try limiting your ActiveRecords to persistence behavior. Sprinkle in some of these techniques to spread out the logic in your domain model and the result will be a much more maintainable application.
You’ll also note that many of the patterns described here are quite simple. The objects are just “Plain Old Ruby Objects” (PORO) used in different ways. And that’s part of the point and the beauty of OOP. Every problem doesn’t need to be solved by a framework or library, and naming matters a great deal.
What do you think of the seven techniques I presented above? What are your favorites and why? Have I missed any? Let me know in the comments!
P.S. If you found this post useful, you may want to subscribe to our email newsletter (see below). It’s low volume, and includes content about OOP and refactoring Rails applications like this.
- Objects on Rails
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Thank you to Steven Bristol, Piotr Solnica, Don Morrison, Jason Roelofs, Giles Bowkett, Justin Ko, Ernie Miller, Steve Klabnik, Pat Maddox, Sergey Nartimov and Nick Gauthier for reviewing this post.