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Showing content from https://github.com/rubocop/rails-style-guide below:

rubocop/rails-style-guide: A community-driven Ruby on Rails style guide

By Bozhidar Batsov

Role models are important.

 — Officer Alex J. Murphy / RoboCop

The goal of this guide is to present a set of best practices and style prescriptions for Ruby on Rails development. It’s a complementary guide to the already existing community-driven Ruby coding style guide.

This Rails style guide recommends best practices so that real-world Rails programmers can write code that can be maintained by other real-world Rails programmers. A style guide that reflects real-world usage gets used, and a style guide that holds to an ideal that has been rejected by the people it is supposed to help risks not getting used at all - no matter how good it is.

The guide is separated into several sections of related rules. I’ve tried to add the rationale behind the rules (if it’s omitted I’ve assumed it’s pretty obvious).

I didn’t come up with all the rules out of nowhere - they are mostly based on my extensive career as a professional software engineer, feedback and suggestions from members of the Rails community and various highly regarded Rails programming resources.

Note

You can generate a PDF copy of this guide using AsciiDoctor PDF, and an HTML copy with AsciiDoctor using the following commands:

# Generates README.pdf
asciidoctor-pdf -a allow-uri-read README.adoc

# Generates README.html
asciidoctor README.adoc

Tip

Install the rouge gem to get nice syntax highlighting in the generated document.

Translations of the guide are available in the following languages:

Tip

Put custom initialization code in config/initializers. The code in initializers executes on application startup.

Keep initialization code for each gem in a separate file with the same name as the gem, for example carrierwave.rb, active_admin.rb, etc.

Adjust accordingly the settings for development, test and production environment (in the corresponding files under config/environments/)

Mark additional assets for precompilation (if any):

# config/environments/production.rb
# Precompile additional assets (application.js, application.css,
#and all non-JS/CSS are already added)
config.assets.precompile += %w( rails_admin/rails_admin.css rails_admin/rails_admin.js )

Keep configuration that’s applicable to all environments in the config/application.rb file.

When upgrading to a newer Rails version, your application’s configuration setting will remain on the previous version. To take advantage of the latest recommended Rails practices, the config.load_defaults setting should match your Rails version.

# good
config.load_defaults 6.1

Avoid creating additional environment configurations than the defaults of development, test and production. If you need a production-like environment such as staging, use environment variables for configuration options.

Keep any additional configuration in YAML files under the config/ directory.

Since Rails 4.2 YAML configuration files can be easily loaded with the new config_for method:

Rails::Application.config_for(:yaml_file)

When you need to add more actions to a RESTful resource (do you really need them at all?) use member and collection routes.

# bad
get 'subscriptions/:id/unsubscribe'
resources :subscriptions

# good
resources :subscriptions do
  get 'unsubscribe', on: :member
end

# bad
get 'photos/search'
resources :photos

# good
resources :photos do
  get 'search', on: :collection
end

If you need to define multiple member/collection routes use the alternative block syntax.

resources :subscriptions do
  member do
    get 'unsubscribe'
    # more routes
  end
end

resources :photos do
  collection do
    get 'search'
    # more routes
  end
end

Use nested routes to express better the relationship between Active Record models.

class Post < ApplicationRecord
  has_many :comments
end

class Comment < ApplicationRecord
  belongs_to :post
end

# routes.rb
resources :posts do
  resources :comments
end

If you need to nest routes more than 1 level deep then use the shallow: true option. This will save user from long URLs posts/1/comments/5/versions/7/edit and you from long URL helpers edit_post_comment_version.

resources :posts, shallow: true do
  resources :comments do
    resources :versions
  end
end

Use namespaced routes to group related actions.

namespace :admin do
  # Directs /admin/products/* to Admin::ProductsController
  # (app/controllers/admin/products_controller.rb)
  resources :products
end

Never use the legacy wild controller route. This route will make all actions in every controller accessible via GET requests.

# very bad
match ':controller(/:action(/:id(.:format)))'

Don’t use match to define any routes unless there is need to map multiple request types among [:get, :post, :patch, :put, :delete] to a single action using :via option.

Keep the controllers skinny - they should only retrieve data for the view layer and shouldn’t contain any business logic (all the business logic should naturally reside in the model).

Each controller action should (ideally) invoke only one method other than an initial find or new.

Minimize the number of instance variables passed between a controller and a view.

Controller actions specified in the option of Action Filter should be in lexical scope. The ActionFilter specified for an inherited action makes it difficult to understand the scope of its impact on that action.

# bad
class UsersController < ApplicationController
  before_action :require_login, only: :export
end

# good
class UsersController < ApplicationController
  before_action :require_login, only: :export

  def export
  end
end

Prefer using a template over inline rendering.

# very bad
class ProductsController < ApplicationController
  def index
    render inline: "<% products.each do |p| %><p><%= p.name %></p><% end %>", type: :erb
  end
end

# good
## app/views/products/index.html.erb
<%= render partial: 'product', collection: products %>

## app/views/products/_product.html.erb
<p><%= product.name %></p>
<p><%= product.price %></p>

## app/controllers/products_controller.rb
class ProductsController < ApplicationController
  def index
    render :index
  end
end

Prefer render plain: over render text:.

# bad - sets MIME type to `text/html`
...
render text: 'Ruby!'
...

# bad - requires explicit MIME type declaration
...
render text: 'Ruby!', content_type: 'text/plain'
...

# good - short and precise
...
render plain: 'Ruby!'
...

Prefer corresponding symbols to numeric HTTP status codes. They are meaningful and do not look like "magic" numbers for less known HTTP status codes.

# bad
...
render status: 403
...

# good
...
render status: :forbidden
...

Introduce non-Active Record model classes freely.

Name the models with meaningful (but short) names without abbreviations.

If you need objects that support ActiveRecord-like behavior (like validations) without the database functionality, use ActiveModel::Model.

class Message
  include ActiveModel::Model

  attr_accessor :name, :email, :content, :priority

  validates :name, presence: true
  validates :email, format: { with: /\A[-a-z0-9_+\.]+\@([-a-z0-9]+\.)+[a-z0-9]{2,4}\z/i }
  validates :content, length: { maximum: 500 }
end

Starting with Rails 6.1, you can also extend the attributes API from ActiveRecord using ActiveModel::Attributes.

class Message
  include ActiveModel::Model
  include ActiveModel::Attributes

  attribute :name, :string
  attribute :email, :string
  attribute :content, :string
  attribute :priority, :integer

  validates :name, presence: true
  validates :email, format: { with: /\A[-a-z0-9_+\.]+\@([-a-z0-9]+\.)+[a-z0-9]{2,4}\z/i }
  validates :content, length: { maximum: 500 }
end

Unless they have some meaning in the business domain, don’t put methods in your model that just format your data (like code generating HTML). These methods are most likely going to be called from the view layer only, so their place is in helpers. Keep your models for business logic and data-persistence only.

Avoid altering Active Record defaults (table names, primary key, etc) unless you have a very good reason (like a database that’s not under your control).

# bad - don't do this if you can modify the schema
class Transaction < ApplicationRecord
  self.table_name = 'order'
  ...
end

Avoid setting ignored_columns. It may overwrite previous assignments and that is almost always a mistake. Prefer appending to the list instead.

class Transaction < ApplicationRecord
  # bad - it may overwrite previous assignments
  self.ignored_columns = %i[legacy]

  # good - the value is appended to the list
  self.ignored_columns += %i[legacy]
  ...
end

Prefer using the hash syntax for enum. Array makes the database values implicit & any insertion/removal/rearrangement of values in the middle will most probably lead to broken code.

class Transaction < ApplicationRecord
  # bad - implicit values - ordering matters
  enum type: %i[credit debit]

  # good - explicit values - ordering does not matter
  enum type: {
    credit: 0,
    debit: 1
  }
end

Group macro-style methods (has_many, validates, etc) in the beginning of the class definition.

class User < ApplicationRecord
  # keep the default scope first (if any)
  default_scope { where(active: true) }

  # constants come up next
  COLORS = %w(red green blue)

  # afterwards we put attr related macros
  attr_accessor :formatted_date_of_birth

  attr_accessible :login, :first_name, :last_name, :email, :password

  # Rails 4+ enums after attr macros
  enum role: { user: 0, moderator: 1, admin: 2 }

  # followed by association macros
  belongs_to :country

  has_many :authentications, dependent: :destroy

  # and validation macros
  validates :email, presence: true
  validates :username, presence: true
  validates :username, uniqueness: { case_sensitive: false }
  validates :username, format: { with: /\A[A-Za-z][A-Za-z0-9._-]{2,19}\z/ }
  validates :password, format: { with: /\A\S{8,128}\z/, allow_nil: true }

  # next we have callbacks
  before_save :cook
  before_save :update_username_lower

  # other macros (like devise's) should be placed after the callbacks

  ...
end

Prefer has_many :through to has_and_belongs_to_many. Using has_many :through allows additional attributes and validations on the join model.

# not so good - using has_and_belongs_to_many
class User < ApplicationRecord
  has_and_belongs_to_many :groups
end

class Group < ApplicationRecord
  has_and_belongs_to_many :users
end

# preferred way - using has_many :through
class User < ApplicationRecord
  has_many :memberships
  has_many :groups, through: :memberships
end

class Membership < ApplicationRecord
  belongs_to :user
  belongs_to :group
end

class Group < ApplicationRecord
  has_many :memberships
  has_many :users, through: :memberships
end

Prefer self[:attribute] over read_attribute(:attribute).

# bad
def amount
  read_attribute(:amount) * 100
end

# good
def amount
  self[:amount] * 100
end

Prefer self[:attribute] = value over write_attribute(:attribute, value).

# bad
def amount
  write_attribute(:amount, 100)
end

# good
def amount
  self[:amount] = 100
end

Always use the "new-style" validations.

# bad
validates_presence_of :email
validates_length_of :email, maximum: 100

# good
validates :email, presence: true, length: { maximum: 100 }

When naming custom validation methods, adhere to the simple rules:

• validate :method_name reads like a natural statement

• the method name explains what it checks

• the method is recognizable as a validation method by its name, not a predicate method

# good
validate :expiration_date_cannot_be_in_the_past
validate :discount_cannot_be_greater_than_total_value
validate :ensure_same_topic_is_chosen

# also good - explicit prefix
validate :validate_birthday_in_past
validate :validate_sufficient_quantity
validate :must_have_owner_with_no_other_items
validate :must_have_shipping_units

# bad
validate :birthday_in_past
validate :owner_has_no_other_items

To make validations easy to read, don’t list multiple attributes per validation.

# bad
validates :email, :password, presence: true
validates :email, length: { maximum: 100 }

# good
validates :email, presence: true, length: { maximum: 100 }
validates :password, presence: true

When a custom validation is used more than once or the validation is some regular expression mapping, create a custom validator file.

# bad
class Person
  validates :email, format: { with: /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i }
end

# good
class EmailValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    record.errors[attribute] << (options[:message] || 'is not a valid email') unless value =~ /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i
  end
end

class Person
  validates :email, email: true
end

Keep custom validators under app/validators.

Consider extracting custom validators to a shared gem if you’re maintaining several related apps or the validators are generic enough.

Use named scopes freely.

class User < ApplicationRecord
  scope :active, -> { where(active: true) }
  scope :inactive, -> { where(active: false) }

  scope :with_orders, -> { joins(:orders).select('distinct(users.id)') }
end

When a named scope defined with a lambda and parameters becomes too complicated, it is preferable to make a class method instead which serves the same purpose of the named scope and returns an ActiveRecord::Relation object. Arguably you can define even simpler scopes like this.

class User < ApplicationRecord
  def self.with_orders
    joins(:orders).select('distinct(users.id)')
  end
end

Order callback declarations in the order in which they will be executed. For reference, see Available Callbacks.

# bad
class Person
  after_commit :after_commit_callback
  before_validation :before_validation_callback
end

# good
class Person
  before_validation :before_validation_callback
  after_commit :after_commit_callback
end

Beware of the behavior of the following methods. They do not run the model validations and could easily corrupt the model state.

# bad
Article.first.decrement!(:view_count)
DiscussionBoard.decrement_counter(:post_count, 5)
Article.first.increment!(:view_count)
DiscussionBoard.increment_counter(:post_count, 5)
person.toggle :active
product.touch
Billing.update_all("category = 'authorized', author = 'David'")
user.update_attribute(:website, 'example.com')
user.update_columns(last_request_at: Time.current)
Post.update_counters 5, comment_count: -1, action_count: 1

# good
user.update_attributes(website: 'example.com')

Use user-friendly URLs. Show some descriptive attribute of the model in the URL rather than its id. There is more than one way to achieve this.

This method is used by Rails for constructing a URL to the object. The default implementation returns the id of the record as a String. It could be overridden to include another human-readable attribute.

class Person
  def to_param
    "#{id} #{name}".parameterize
  end
end

In order to convert this to a URL-friendly value, parameterize should be called on the string. The id of the object needs to be at the beginning so that it can be found by the find method of Active Record.

It allows creation of human-readable URLs by using some descriptive attribute of the model instead of its id.

class Person
  extend FriendlyId
  friendly_id :name, use: :slugged
end

Check the gem documentation for more information about its usage.

Use find_each to iterate over a collection of AR objects. Looping through a collection of records from the database (using the all method, for example) is very inefficient since it will try to instantiate all the objects at once. In that case, batch processing methods allow you to work with the records in batches, thereby greatly reducing memory consumption.

# bad
Person.all.each do |person|
  person.do_awesome_stuff
end

Person.where('age > 21').each do |person|
  person.party_all_night!
end

# good
Person.find_each do |person|
  person.do_awesome_stuff
end

Person.where('age > 21').find_each do |person|
  person.party_all_night!
end

Since Rails creates callbacks for dependent associations, always call before_destroy callbacks that perform validation with prepend: true.

# bad (roles will be deleted automatically even if super_admin? is true)
has_many :roles, dependent: :destroy

before_destroy :ensure_deletable

def ensure_deletable
  raise "Cannot delete super admin." if super_admin?
end

# good
has_many :roles, dependent: :destroy

before_destroy :ensure_deletable, prepend: true

def ensure_deletable
  raise "Cannot delete super admin." if super_admin?
end

Define the dependent option to the has_many and has_one associations.

# bad
class Post < ApplicationRecord
  has_many :comments
end

# good
class Post < ApplicationRecord
  has_many :comments, dependent: :destroy
end

When persisting AR objects always use the exception raising bang! method or handle the method return value. This applies to create, save, update, destroy, first_or_create and find_or_create_by.

# bad
user.create(name: 'Bruce')

# bad
user.save

# good
user.create!(name: 'Bruce')
# or
bruce = user.create(name: 'Bruce')
if bruce.persisted?
  ...
else
  ...
end

# good
user.save!
# or
if user.save
  ...
else
  ...
end

Avoid string interpolation in queries, as it will make your code susceptible to SQL injection attacks.

# bad - param will be interpolated unescaped
Client.where("orders_count = #{params[:orders]}")

# good - param will be properly escaped
Client.where('orders_count = ?', params[:orders])

Consider using named placeholders instead of positional placeholders when you have more than 1 placeholder in your query.

# okish
Client.where(
  'orders_count >= ? AND country_code = ?',
  params[:min_orders_count], params[:country_code]
)

# good
Client.where(
  'orders_count >= :min_orders_count AND country_code = :country_code',
  min_orders_count: params[:min_orders_count], country_code: params[:country_code]
)

Prefer find over where.take!, find_by!, and find_by_id! when you need to retrieve a single record by primary key id and raise ActiveRecord::RecordNotFound when the record is not found.

# bad
User.where(id: id).take!

# bad
User.find_by_id!(id)

# bad
User.find_by!(id: id)

# good
User.find(id)

Prefer find_by over where.take and find_by_attribute when you need to retrieve a single record by one or more attributes and return nil when the record is not found.

# bad
User.where(email: email).take
User.where(first_name: 'Bruce', last_name: 'Wayne').take

# bad
User.find_by_email(email)
User.find_by_first_name_and_last_name('Bruce', 'Wayne')

# good
User.find_by(email: email)
User.find_by(first_name: 'Bruce', last_name: 'Wayne')

Prefer passing conditions to where and where.not as a hash over using fragments of SQL.

# bad
User.where("name = ?", name)

# good
User.where(name: name)

# bad
User.where("id != ?", id)

# good
User.where.not(id: id)

If you’re using Rails 6.1 or higher, use where.missing to find missing relationship records.

# bad
Post.left_joins(:author).where(authors: { id: nil })

# good
Post.where.missing(:author)

Don’t use the id column for ordering. The sequence of ids is not guaranteed to be in any particular order, despite often (incidentally) being chronological. Use a timestamp column to order chronologically. As a bonus the intent is clearer.

# bad
scope :chronological, -> { order(id: :asc) }

# good
scope :chronological, -> { order(created_at: :asc) }

Use pluck to select a single value from multiple records.

# bad
User.all.map(&:name)

# bad
User.all.map { |user| user[:name] }

# good
User.pluck(:name)

Use pick to select a single value from a single record.

# bad
User.pluck(:name).first

# bad
User.first.name

# good
User.pick(:name)

Prefer ids over pluck(:id).

# bad
User.pluck(:id)

# good
User.ids

When specifying an explicit query in a method such as find_by_sql, use heredocs with squish. This allows you to legibly format the SQL with line breaks and indentations, while supporting syntax highlighting in many tools (including GitHub, Atom, and RubyMine).

User.find_by_sql(<<-SQL.squish)
  SELECT
    users.id, accounts.plan
  FROM
    users
  INNER JOIN
    accounts
  ON
    accounts.user_id = users.id
  # further complexities...
SQL

String#squish removes the indentation and newline characters so that your server log shows a fluid string of SQL rather than something like this:

SELECT\n    users.id, accounts.plan\n  FROM\n    users\n  INNER JOIN\n    accounts\n  ON\n    accounts.user_id = users.id

When querying Active Record collections, prefer size (selects between count/length behavior based on whether collection is already loaded) or length (always loads the whole collection and counts the array elements) over count (always does a database query for the count).

# bad
User.count

# good
User.all.size

# good - if you really need to load all users into memory
User.all.length

Use ranges instead of defining comparative conditions using a template for scalar values.

# bad
User.where("created_at >= ?", 30.days.ago).where("created_at <= ?", 7.days.ago)
User.where("created_at >= ? AND created_at <= ?", 30.days.ago, 7.days.ago)
User.where("created_at >= :start AND created_at <= :end", start: 30.days.ago, end: 7.days.ago)

# good
User.where(created_at: 30.days.ago..7.days.ago)

# bad
User.where("created_at >= ?", 7.days.ago)

# good
User.where(created_at: 7.days.ago..)

# note - ranges are inclusive or exclusive of their ending, not beginning
User.where(created_at: 7.days.ago..)  # produces >=
User.where(created_at: 7.days.ago...) # also produces >=
User.where(created_at: ..7.days.ago)  # inclusive: produces <=
User.where(created_at: ...7.days.ago) # exclusive: produces <

# okish - there is no range syntax that would denote exclusion at the beginning of the range
Customer.where("purchases_count > :min AND purchases_count <= :max", min: 0, max: 5)

Note

Avoid passing multiple attributes to where.not. Rails logic in this case has changed in Rails 6.1 and will now yield results matching either of those conditions, e.g. where.not(status: 'active', plan: 'basic') would return records with active status when the plan is business.

# bad
User.where.not(status: 'active', plan: 'basic')

# good
User.where.not('status = ? AND plan = ?', 'active', 'basic')

Using all as a receiver is redundant. The result won’t change without all, so it should be removed.

# bad
User.all.find(id)
User.all.order(:created_at)
users.all.where(id: ids)
user.articles.all.order(:created_at)

# good
User.find(id)
User.order(:created_at)
users.where(id: ids)
user.articles.order(:created_at)

Note

The following methods behave differently without all:

So, when considering removing all from the receiver of these methods, it is recommended to refer to the documentation to understand how the behavior changes.

Keep the schema.rb (or structure.sql) under version control.

Use rake db:schema:load instead of rake db:migrate to initialize an empty database.

Enforce default values in the migrations themselves instead of in the application layer.

# bad - application enforced default value
class Product < ApplicationRecord
  def amount
    self[:amount] || 0
  end
end

# good - database enforced
class AddDefaultAmountToProducts < ActiveRecord::Migration
  def change
    change_column_default :products, :amount, 0
  end
end

While enforcing table defaults only in Rails is suggested by many Rails developers, it’s an extremely brittle approach that leaves your data vulnerable to many application bugs. And you’ll have to consider the fact that most non-trivial apps share a database with other applications, so imposing data integrity from the Rails app is impossible.

With SQL databases, if a boolean column is not given a default value, it will have three possible values: true, false and NULL. Boolean operators work in unexpected ways with NULL.

For example in SQL queries, true AND NULL is NULL (not false), true AND NULL OR false is NULL (not false). This can make SQL queries return unexpected results.

To avoid such situations, boolean columns should always have a default value and a NOT NULL constraint.

# bad - boolean without a default value
add_column :users, :active, :boolean

# good - boolean with a default value (`false` or `true`) and with restricted `NULL`
add_column :users, :active, :boolean, default: true, null: false
add_column :users, :admin, :boolean, default: false, null: false

Enforce foreign-key constraints. As of Rails 4.2, Active Record supports foreign key constraints natively.

# bad - does not add foreign keys
create_table :comment do |t|
  t.references :article
  t.belongs_to :user
  t.integer :category_id
end

# good
create_table :comment do |t|
  t.references :article, foreign_key: true
  t.belongs_to :user, foreign_key: true
  t.references :category, foreign_key: { to_table: :comment_categories }
end

When writing constructive migrations (adding tables or columns), use the change method instead of up and down methods.

# the old way
class AddNameToPeople < ActiveRecord::Migration
  def up
    add_column :people, :name, :string
  end

  def down
    remove_column :people, :name
  end
end

# the new preferred way
class AddNameToPeople < ActiveRecord::Migration
  def change
    add_column :people, :name, :string
  end
end

If you have to use models in migrations, make sure you define them so that you don’t end up with broken migrations in the future.

# db/migrate/<migration_file_name>.rb
# frozen_string_literal: true

# bad
class ModifyDefaultStatusForProducts < ActiveRecord::Migration
  def change
    old_status = 'pending_manual_approval'
    new_status = 'pending_approval'

    reversible do |dir|
      dir.up do
        Product.where(status: old_status).update_all(status: new_status)
        change_column :products, :status, :string, default: new_status
      end

      dir.down do
        Product.where(status: new_status).update_all(status: old_status)
        change_column :products, :status, :string, default: old_status
      end
    end
  end
end

# good
# Define `table_name` in a custom named class to make sure that you run on the
# same table you had during the creation of the migration.
# In future if you override the `Product` class and change the `table_name`,
# it won't break the migration or cause serious data corruption.
class MigrationProduct < ActiveRecord::Base
  self.table_name = :products
end

class ModifyDefaultStatusForProducts < ActiveRecord::Migration
  def change
    old_status = 'pending_manual_approval'
    new_status = 'pending_approval'

    reversible do |dir|
      dir.up do
        MigrationProduct.where(status: old_status).update_all(status: new_status)
        change_column :products, :status, :string, default: new_status
      end

      dir.down do
        MigrationProduct.where(status: new_status).update_all(status: old_status)
        change_column :products, :status, :string, default: old_status
      end
    end
  end
end

Name your foreign keys explicitly instead of relying on Rails auto-generated FK names. (https://guides.rubyonrails.org/active_record_migrations.html#foreign-keys)

# bad
class AddFkArticlesToAuthors < ActiveRecord::Migration
  def change
    add_foreign_key :articles, :authors
  end
end

# good
class AddFkArticlesToAuthors < ActiveRecord::Migration
  def change
    add_foreign_key :articles, :authors, name: :articles_author_id_fk
  end
end

Don’t use non-reversible migration commands in the change method. Reversible migration commands are listed below. ActiveRecord::Migration::CommandRecorder

# bad
class DropUsers < ActiveRecord::Migration
  def change
    drop_table :users
  end
end

# good
class DropUsers < ActiveRecord::Migration
  def up
    drop_table :users
  end

  def down
    create_table :users do |t|
      t.string :name
    end
  end
end

# good
# In this case, block will be used by create_table in rollback
# https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters.html#method-i-drop_table
class DropUsers < ActiveRecord::Migration
  def change
    drop_table :users do |t|
      t.string :name
    end
  end
end

Never call the model layer directly from a view.

Avoid complex formatting in the views. A view helper is useful for simple cases, but if it’s more complex then consider using a decorator or presenter.

Mitigate code duplication by using partial templates and layouts.

Avoid using instance variables in partials, pass a local variable to render instead. The partial may be used in a different controller or action, where the variable can have a different name or even be absent. In these cases, an undefined instance variable will not raise an exception whereas a local variable will.

<!-- bad -->
<!-- app/views/courses/show.html.erb -->
<%= render 'course_description' %>
<!-- app/views/courses/_course_description.html.erb -->
<%= @course.description %>

<!-- good -->
<!-- app/views/courses/show.html.erb -->
<%= render 'course_description', course: @course %>
<!-- app/views/courses/_course_description.html.erb -->
<%= course.description %>

No strings or other locale specific settings should be used in the views, models and controllers. These texts should be moved to the locale files in the config/locales directory.

When the labels of an Active Record model need to be translated, use the activerecord scope:

en:
  activerecord:
    models:
      user: Member
    attributes:
      user:
        name: 'Full name'

Then User.model_name.human will return "Member" and User.human_attribute_name("name") will return "Full name". These translations of the attributes will be used as labels in the views.

Separate the texts used in the views from translations of Active Record attributes. Place the locale files for the models in a folder locales/models and the texts used in the views in folder locales/views.

When organization of the locale files is done with additional directories, these directories must be described in the application.rb file in order to be loaded.

# config/application.rb
config.i18n.load_path += Dir[Rails.root.join('config', 'locales', '**', '*.{rb,yml}')]

Place the shared localization options, such as date or currency formats, in files under the root of the locales directory.

Use the short form of the I18n methods: I18n.t instead of I18n.translate and I18n.l instead of I18n.localize.

Use "lazy" lookup for locale entries from views and controllers. Let’s say we have the following structure:

en:
  users:
    show:
      title: 'User details page'

The value for users.show.title can be looked up in the template app/views/users/show.html.haml like this:

# bad
= t 'users.show.title'

# good
= t '.title'

Use dot-separated locale keys instead of specifying the :scope option with an array or a single symbol. Dot-separated notation is easier to read and trace the hierarchy.

# bad
I18n.t :record_invalid, scope: [:activerecord, :errors, :messages]

# good
I18n.t :record_invalid, scope: 'activerecord.errors.messages'
I18n.t 'activerecord.errors.messages.record_invalid'

# bad
I18n.t :title, scope: :invitation

# good
I18n.t 'title.invitation'

More detailed information about the Rails I18n can be found in the Rails Guides

Use the asset pipeline to leverage organization within your application.

Reserve app/assets for custom stylesheets, javascripts, or images.

Use lib/assets for your own libraries that don’t really fit into the scope of the application.

Third party code such as jQuery or bootstrap should be placed in vendor/assets.

Name the mailers SomethingMailer. Without the Mailer suffix it isn’t immediately apparent what’s a mailer and which views are related to the mailer.

Provide both HTML and plain-text view templates.

Enable errors raised on failed mail delivery in your development environment. The errors are disabled by default.

# config/environments/development.rb

config.action_mailer.raise_delivery_errors = true

Use a local SMTP server like Mailcatcher in development environment.

# config/environments/development.rb

config.action_mailer.smtp_settings = {
  address: 'localhost',
  port: 1025,
  # more settings
}

Provide default settings for the host name.

# config/environments/development.rb
config.action_mailer.default_url_options = { host: "#{local_ip}:3000" }

# config/environments/production.rb
config.action_mailer.default_url_options = { host: 'your_site.com' }

# in your mailer class
default_url_options[:host] = 'your_site.com'

Format the from and to addresses properly. Use the following format:

# in your mailer class
default from: 'Your Name <info@your_site.com>'

If you’re using Rails 6.1 or higher, you can use the email_address_with_name method:

# in your mailer class
default from: email_address_with_name('info@your_site.com', 'Your Name')

Make sure that the e-mail delivery method for your test environment is set to test:

# config/environments/test.rb

config.action_mailer.delivery_method = :test

The delivery method for development and production should be smtp:

# config/environments/development.rb, config/environments/production.rb

config.action_mailer.delivery_method = :smtp

When sending html emails all styles should be inline, as some mail clients have problems with external styles. This however makes them harder to maintain and leads to code duplication. There are two similar gems that transform the styles and put them in the corresponding html tags: premailer-rails and roadie.

Sending emails while generating page response should be avoided. It causes delays in loading of the page and request can timeout if multiple email are sent. To overcome this emails can be sent in background process with the help of sidekiq gem.

Prefer Ruby 2.3’s safe navigation operator &. over ActiveSupport#try!.

# bad
obj.try! :fly

# good
obj&.fly

Prefer Ruby’s Standard Library methods over ActiveSupport aliases.

# bad
'the day'.starts_with? 'th'
'the day'.ends_with? 'ay'

# good
'the day'.start_with? 'th'
'the day'.end_with? 'ay'

Prefer Ruby’s Standard Library over uncommon Active Support extensions.

# bad
(1..50).to_a.forty_two
1.in? [1, 2]
'day'.in? 'the day'

# good
(1..50).to_a[41]
[1, 2].include? 1
'the day'.include? 'day'

Prefer Ruby’s comparison operators over Active Support’s Array#inquiry, and String#inquiry.

# bad - String#inquiry
ruby = 'two'.inquiry
ruby.two?

# good
ruby = 'two'
ruby == 'two'

# bad - Array#inquiry
pets = %w(cat dog).inquiry
pets.gopher?

# good
pets = %w(cat dog)
pets.include? 'cat'

Prefer Active Support’s exclude? over Ruby’s negated include?.

# bad
!array.include?(2)
!hash.include?(:key)
!string.include?('substring')

# good
array.exclude?(2)
hash.exclude?(:key)
string.exclude?('substring')

If you’re using Ruby 2.3 or higher, prefer squiggly heredoc (<<~) over Active Support’s strip_heredoc.

# bad
<<EOS.strip_heredoc
  some text
EOS

# bad
<<-EOS.strip_heredoc
  some text
EOS

# good
<<~EOS
  some text
EOS

If you’re using Rails 7.0 or higher, prefer to_fs over to_formatted_s. to_formatted_s is just too cumbersome for a method used that frequently.

# bad
time.to_formatted_s(:db)
date.to_formatted_s(:db)
datetime.to_formatted_s(:db)
42.to_formatted_s(:human)

# good
time.to_fs(:db)
date.to_fs(:db)
datetime.to_fs(:db)
42.to_fs(:human)

Configure your timezone accordingly in application.rb.

config.time_zone = 'Eastern European Time'
# optional - note it can be only :utc or :local (default is :utc)
config.active_record.default_timezone = :local

Don’t use Time.parse.

# bad
Time.parse('2015-03-02 19:05:37') # => Will assume time string given is in the system's time zone.

# good
Time.zone.parse('2015-03-02 19:05:37') # => Mon, 02 Mar 2015 19:05:37 EET +02:00

Don’t use String#to_time

# bad - assumes time string given is in the system's time zone.
'2015-03-02 19:05:37'.to_time

# good
Time.zone.parse('2015-03-02 19:05:37') # => Mon, 02 Mar 2015 19:05:37 EET +02:00

Don’t use Time.now.

# bad
Time.now # => Returns system time and ignores your configured time zone.

# good
Time.zone.now # => Fri, 12 Mar 2014 22:04:47 EET +02:00
Time.current # Same thing but shorter.

Prefer all_(day|week|month|quarter|year) over beginning_of_(day|week|month|quarter|year)..end_of_(day|week|month|quarter|year) to get the range of date/time.

# bad
date.beginning_of_day..date.end_of_day
date.beginning_of_week..date.end_of_week
date.beginning_of_month..date.end_of_month
date.beginning_of_quarter..date.end_of_quarter
date.beginning_of_year..date.end_of_year

# good
date.all_day
date.all_week
date.all_month
date.all_quarter
date.all_year

If used without a parameter, prefer from_now and ago instead of since, after, until or before.

# bad - It's not clear that the qualifier refers to the current time (which is the default parameter)
5.hours.since
5.hours.after
5.hours.before
5.hours.until

# good
5.hours.from_now
5.hours.ago

If used with a parameter, prefer since, after, until or before instead of from_now and ago.

# bad - It's confusing and misleading to read
2.days.from_now(yesterday)
2.days.ago(yesterday)

# good
2.days.since(yesterday)
2.days.after(yesterday)
2.days.before(yesterday)
2.days.until(yesterday)

Avoid using negative numbers for the duration subject. Always prefer using a qualifier that allows using positive literal numbers.

# bad - It's confusing and misleading to read
-5.hours.from_now
-5.hours.ago

# good
5.hours.ago
5.hours.from_now

Use Duration methods instead of adding and subtracting with the current time.

# bad
Time.current - 1.minute
Time.zone.now + 2.days

# good
1.minute.ago
2.days.from_now

Put gems used only for development or testing in the appropriate group in the Gemfile.

Use only established gems in your projects. If you’re contemplating on including some little-known gem you should do a careful review of its source code first.

Do not remove the Gemfile.lock from version control. This is not some randomly generated file - it makes sure that all of your team members get the same gem versions when they do a bundle install.

Prefer integration style controller tests over functional style controller tests, as recommended in the Rails documentation.

# bad
class MyControllerTest < ActionController::TestCase
end

# good
class MyControllerTest < ActionDispatch::IntegrationTest
end

If your projects depends on various external processes use foreman to manage them.

There are a few excellent resources on Rails style, that you should consider if you have time to spare:

Nothing written in this guide is set in stone. It’s my desire to work together with everyone interested in Rails coding style, so that we could ultimately create a resource that will be beneficial to the entire Ruby community.

Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!

You can also support the project (and RuboCop) with financial contributions via Patreon.

It’s easy, just follow the contribution guidelines below:

A community-driven style guide is of little use to a community that doesn’t know about its existence. Tweet about the guide, share it with your friends and colleagues. Every comment, suggestion or opinion we get makes the guide just a little bit better. And we want to have the best possible guide, don’t we?

Cheers,
Bozhidar

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