Different ways to apply custom layouts to a controller's actions

custom_layout.rb

# Passing a String to the layout:
class TestsController < ApplicationController
  layout "test_layout" # maps to app/views/layout/test_layout.html.[erb/haml/slim/whatever template engine you're using]
  
  # The test_layout is going to be applied here...
  def index
  end
  
  # ...and here as well
  def new
  end
end

# Passing a Symbol to the layout:
class TestsController < ApplicationController
  # Is going to run the my_custom_method when it needs to
  # define the layout for the current view.
  # It's useful for defining the layout dynamically, for
  # example when it depends on some parameter or on the user's role.
  layout :my_custom_method
  
  # Is going to render whatever layout is returned by my_custom_method
  def index
  end
  
  # Here too!
  def new
  end
  
  # It's a good practice to use the private visibility for methods that are not actions in controllers.
  private
  
  def my_custom_method
    if current_user.admin? # chooses the layout according to the user role in the application
      "admin_layout"
    else
      "customer_layout"
    end
  end
end

# Setting an action-dependent layout:
class TestsController < ApplicationController
  # This action is going to use the app/views/layouts/test_layout.html.erb layout
  def index
    # Don't pass a Symbol here (render layout: :test_layout),
    # is not going to map to a method as in the previous example,
    # but raise an ArgumentError exception with the following message:
    # String, Proc, :default, true, or false, expected for `layout'; you passed :test_layout
    render layout: "test_layout"
  end
  
  # This action is going to use the app/views/layouts/application.html.erb layout
  def new
  end
end