Traits and partial classes for Ruby
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Modularity 2 - Traits and partial classes for Ruby

Modularity enhances Ruby's Module so it can be used traits and partial classes. This allows very simple definition of meta-programming macros like the has_many that you know from Rails.

Modularity also lets you organize large models into multiple source files in a way that is less awkward than using modules.

Note that this is Modularity 2, which has a different syntax older version. Modularity 1 users can use a script to migrate their code or use the modularity1 branch.

Example 1: Easy meta-programming macros

Ruby allows you to construct classes using meta-programming macros like acts_as_tree or has_many :items. These macros will add methods, callbacks, etc. to the calling class. However, right now Ruby (and Rails) makes it awkward to define such macros in your project as part of your application domain.

Modularity allows you to extract common behaviour into reusable macros by defining traits with parameters. Your macros can live in your application, allowing you to express your application domain in both classes and macros.

Here is an example of a strip_field macro, which created setter methods that remove leading and trailing whitespace from newly assigned values:

# app/models/article.rb
class Article < ActiveRecord::Base
  include DoesStripFields[:name, :brand]

# app/models/shared/does_strip_fields.rb
module DoesStripFields
  as_trait do |*fields|
    fields.each do |field|
      define_method("#{field}=") do |value|
        self[field] = value.strip

Notice the as_trait block.

We like to add app/models/shared and app/controllers/shared to the load paths of our Rails projects. These are great places to store macros that are re-used from multiple classes.

Example 2: Mixins with class methods

Using a module to add both instance methods and class methods is very awkward. Modularity does away with the clutter and lets you say this:

# app/models/model.rb
class Model
  include Mixin

# app/models/mixin.rb
module Mixin
  as_trait do
    def instance_method
      # ...
    def self.class_method
      # ..

private and protected will also work as expected when defining a trait.

Example 3: Splitting a model into multiple source files

Models are often concerned with multiple themes like "authentication", "contact info" or "permissions", each requiring a couple of validations and callbacks here, and some method there. Modularity lets you organize your model into multiple partial classes, so each file can deal with a single aspect of your model:

# app/models/user.rb
class User < ActiveRecord::Base
  include DoesAuthentication
  include DoesPermissions

# app/models/user/does_authentication.rb
module User::DoesAuthentication
  as_trait do
    # methods, validations, etc. regarding usernames and passwords go here

# app/models/user/does_permissions.rb
module User::DoesPermissions
  as_trait do
    # methods, validations, etc. regarding contact information go here

Some criticism has been raised for splitting large models into files like this. Essentially, even though have an easier time navigating your code, you will still have one giant model with many side effects.

There are many better ways to decompose a huge Ruby class.


Add the following to your Gemfile:

gem 'modularity', '>=2'

Now run bundle install.

Migrating from Modularity 1

If you have been using Modularity 1 with the does syntax, we provide a script to migrate your Ruby project automatically.

  1. Make sure your project has tests and you have a backup of your files (or pushed your commits to Git)

  2. From your project directory, do this:

    find . -name "*.rb" | migrate-modularity1-to-modularity2
  3. The script will rename your files and change your code. It will also syntax-check your files after conversion (since the script is not perfect).

  4. Check the diff to see what the script has done.

  5. Run tests to see if everything still works.


Henning Koch from