Ruby style guide

This is a GitLab-specific style guide for Ruby code. Everything documented in this page can be reopened for discussion.

We use RuboCop to enforce Ruby style guide rules.

Where a RuboCop rule is absent, refer to the following style guides as general guidelines to write idiomatic Ruby:

Generally, if a style is not covered by existing RuboCop rules or the above style guides, it shouldn't be a blocker.

Some styles we have decided no one should not have a strong opinion on.

Styles we have no rule for

These styles are not backed by a RuboCop rule.

For every style added to this section, link the discussion from the section's history note to provide context and serve as a reference.

Instance variable access using `attr_reader`

Instance variables can be accessed in a variety of ways in a class:

ruby

# public
class Foo
  attr_reader :my_var

  def initialize(my_var)
    @my_var = my_var
  end

  def do_stuff
    puts my_var
  end
end

# private
class Foo
  def initialize(my_var)
    @my_var = my_var
  end

  private

  attr_reader :my_var

  def do_stuff
    puts my_var
  end
end

# direct
class Foo
  def initialize(my_var)
    @my_var = my_var
  end

  private

  def do_stuff
    puts @my_var
  end
end

Public attributes should only be used if they are accessed outside of the class. There is not a strong opinion on what strategy is used when attributes are only accessed internally, as long as there is consistency in related code.

Newlines style guide

In addition to the RuboCop's Layout/EmptyLinesAroundMethodBody and Cop/LineBreakAroundConditionalBlock that enforce some newline styles, we have the following guidelines that are not backed by RuboCop.

Rule: separate code with newlines only to group together related logic

ruby

# bad
def method
  issue = Issue.new

  issue.save

  render json: issue
end

ruby

# good
def method
  issue = Issue.new
  issue.save

  render json: issue
end

Rule: newline before block

ruby

# bad
def method
  issue = Issue.new
  if issue.save
    render json: issue
  end
end

ruby

# good
def method
  issue = Issue.new

  if issue.save
    render json: issue
  end
end

Exception: no need for a newline when code block starts or ends right inside another code block

ruby

# bad
def method
  if issue

    if issue.valid?
      issue.save
    end

  end
end

ruby

# good
def method
  if issue
    if issue.valid?
      issue.save
    end
  end
end

Method ordering within classes

For ordering methods at the class level (public, protected, private sections), refer to RuboCop's Layout/ClassStructure.

Within each visibility section, consider the following principles to improve readability:

Rule: Order methods by level of abstraction (high-level before low-level)

Methods should generally be ordered from highest to lowest level of abstraction. This means:

Methods that orchestrate or coordinate work should come first
Helper methods that support those orchestrator methods should come after

This follows the "newspaper style" or "stepdown rule" principle, where code reads like a story from top to bottom, with the most important operations first and implementation details revealed progressively.

ruby

# good - orchestrator method before helpers
class CommitMessageProcessor
  def execute
    other_method_calls
    process_commit_message
  end

  private

  def process_commit_message
    title = extract_title(commit.message)
    body = extract_body(commit.message)

    # ... process title and body
  end

  def extract_title(message)
    message.split("\n").first
  end

  def extract_body(message)
    message.split("\n")[1..]
  end
end

ruby

# bad - helper methods before the method that uses them
class CommitMessageProcessor
  def execute
    other_method_calls
    process_commit_message
  end

  private

  def extract_title(message)
    message.split("\n").first
  end

  def extract_body(message)
    message.split("\n")[1..]
  end

  def process_commit_message
    title = extract_title(commit.message)
    body = extract_body(commit.message)

    # ... process title and body
  end
end

This ordering helps readers understand:

What the code does (by reading the high-level method first)
How it does it (by reading the helper methods after)

Following this ordering pattern helps reviewers and future maintainers understand code flow more quickly, especially in service objects, processors, and other classes with clear orchestration patterns.

Exceptions to this ordering may be appropriate when:

Methods are grouped by a domain concept that's more important than abstraction level
Alphabetical ordering provides significant value for a large number of similar methods

When a class has multiple high-level methods that serve different, unrelated purposes, group each high-level method with its supporting helper methods. Alternatively, consider extracting the implementation into separate service classes where each class has a single clear responsibility.

[!note] Beyond two levels of method calls (a method calling a method calling a method), this pattern can become unwieldy and hard to follow. If you find yourself with deep nesting, consider refactoring into separate classes or simplifying the logic.

Rails / ActiveRecord

This section contains GitLab-specific guidelines for Rails and ActiveRecord usage.

Avoid ActiveRecord callbacks

ActiveRecord callbacks allow you to "trigger logic before or after an alteration of an object's state."

Use callbacks when no superior alternative exists, but employ them only if you thoroughly understand the reasons for doing so.

When adding new lifecycle events for ActiveRecord objects, it is preferable to add the logic to a service class instead of a callback.

Why callbacks should be avoided

In general, callbacks should be avoided because:

Callbacks are hard to reason about because invocation order is not obvious and they break code narrative.
Callbacks are harder to locate and navigate because they rely on reflection to trigger rather than being ordinary method calls.
Callbacks make it difficult to apply changes selectively to an object's state because changes always trigger the entire callback chain.
Callbacks trap logic in the ActiveRecord class. This tight coupling encourages fat models that contain too much business logic, which could instead live in service objects that are more reusable, composable, and are easier to test.
Illegal state transitions of an object can be better enforced through attribute validations.
Heavy use of callbacks affects factory creation speed. With some classes having hundreds of callbacks, creating an instance of that object for an automated test can be a very slow operation, resulting in slow specs.

Some of these examples are discussed in this video from thoughtbot.

The GitLab codebase relies heavily on callbacks and it is hard to refactor them once added due to invisible dependencies. As a result, this guideline does not call for removing all existing callbacks.

When to use callbacks

Callbacks can be used in special cases. Some examples of cases where adding a callback makes sense:

A dependency uses callbacks and we would like to override the callback behavior.
Incrementing cache counts.
Data normalization that only relates to data on the current model.

Example of moving from a callback to a service

There is a project with the following basic data model:

ruby

class Project
  has_one :repository
end

class Repository
  belongs_to :project
end

Say we want to create a repository after a project is created and use the project name as the repository name. A developer familiar with Rails might immediately think: sounds like a job for an ActiveRecord callback! And add this code:

ruby

class Project
  has_one :repository

  after_initialize :create_random_name
  after_create :create_repository

  def create_random_name
    SecureRandom.alphanumeric
  end

  def create_repository
    Repository.create!(project: self)
  end
end

class Repository
  after_initialize :set_name

  def set_name
    name = project.name
  end
end

class ProjectsController
  def create
    Project.create! # also creates a repository and names it
  end
end

While this seems pretty harmless for a baby Rails app, adding this type of logic via callbacks has many downsides once your Rails app becomes large and complex (all of which are listed in this documentation). Instead, we can add this logic to a service class:

ruby

class Project
  has_one :repository
end

class Repository
  belongs_to :project
end

class ProjectCreator
  def self.execute
    ApplicationRecord.transaction do
      name = SecureRandom.alphanumeric
      project = Project.create!(name: name)
      Repository.create!(project: project, name: name)
    end
  end
end

class ProjectsController
  def create
    ProjectCreator.execute
  end
end

With an application this simple, it can be hard to see the benefits of the second approach. But we already some benefits:

Can test Repository creation logic separate from Project creation logic. Code no longer violates law of demeter (Repository class doesn't need to know project.name).
Clarity of invocation order.
Open to change: if we decide there are some scenarios where we do not want a repository created for a project, we can create a new service class rather than needing to refactor to Project and Repository classes.
Each instance of a Project factory does not create a second (Repository) object.

ApplicationRecord / ActiveRecord model scopes

When creating a new scope, consider the following prefixes.

`for_`

For scopes which filter where(belongs_to: record). For example:

ruby

scope :for_project, ->(project) { where(project: project) }
Timelogs.for_project(project)

`with_`

For scopes which joins, includes, or filters where(has_one: record) or where(has_many: record) or where(boolean condition) For example:

ruby

scope :with_labels, -> { includes(:labels) }
AbuseReport.with_labels

scope :with_status, ->(status) { where(status: status) }
Clusters::AgentToken.with_status(:active)

scope :with_due_date, -> { where.not(due_date: nil) }
Issue.with_due_date

It is also fine to use custom scope names, for example:

ruby

scope :undeleted, -> { where('policy_index >= 0') }
Security::Policy.undeleted

`order_by_`

For scopes which order. For example:

ruby

scope :order_by_name, -> { order(:name) }
Namespace.order_by_name

scope :order_by_updated_at, ->(direction = :asc) { order(updated_at: direction) }
Project.order_by_updated_at(:desc)

Styles we have no opinion on

If a RuboCop rule is proposed and we choose not to add it, we should document that decision in this guide so it is more discoverable and link the relevant discussion as a reference.

Quoting string literals

Due to the sheer amount of work to rectify, we do not care whether string literals are single or double-quoted.

Previous discussions include:

Individual groups may choose to have an opinion on consistency of quoting styles within the bounded contexts they own, but these decisions only apply to code within that context.

Type safety

Now that we've upgraded to Ruby 3, we have more options available to enforce type safety.

Some of these options are supported as part of the Ruby syntax and do not require the use of specific type safety tools like Sorbet or RBS. However, we might consider these tools in the future as well.

For now, we can use YARD annotations to define types. IDEs such as RubyMine provide support for YARD when showing type-based inspection errors.

For more information, see Type safety in the remote_development domain README.

Functional patterns

Although Ruby and especially Rails are primarily based on object-oriented programming patterns, Ruby is a very flexible language and supports functional programming patterns as well.

Functional programming patterns, especially in domain logic, can often result in more readable, maintainable, and bug-resistant code while still using idiomatic and familiar Ruby patterns. However, functional programming patterns should be used carefully because some patterns would cause confusion and should be avoided even if they're directly supported by Ruby. The curry method is a likely example.

For more information, see:

Styles we have no rule for

Instance variable access using attr_reader

Newlines style guide

Rule: separate code with newlines only to group together related logic

Rule: newline before block

Exception: no need for a newline when code block starts or ends right inside another code block

Method ordering within classes

Rule: Order methods by level of abstraction (high-level before low-level)

Rails / ActiveRecord

Avoid ActiveRecord callbacks

Why callbacks should be avoided

When to use callbacks

Example of moving from a callback to a service

ApplicationRecord / ActiveRecord model scopes

for_

with_

order_by_

Styles we have no opinion on

Quoting string literals

Type safety

Functional patterns

Instance variable access using `attr_reader`

`for_`

`with_`

`order_by_`