Ruby on Rails: Where to define global constants ?

The “responsible”  constants  create a class methods to access to without creating a new object instance,

class Card < ActiveRecord::Base
   def self.colours
      ['white', 'blue']
   end
end
 
# accessible like this
Card.colours

Create class variables  an accessor. A class variables might act  as inheritance and  multi-thread environments,

class Card < ActiveRecord::Base
  @@colours = ['white', 'blue']
  cattr_reader :colours
end
 
# accessible the same as above

Both options change and  the returned array on each invocation of the accessor method . The unchangeable constant  define as a model class:

class Card < ActiveRecord::Base
  COLOURS = ['white', 'blue'].freeze
end
 
# accessible as
Card::COLOURS

The color are really global and used in more than one model context,

# put this into config/initializers/my_constants.rb
COLOURS = ['white', 'blue'].freeze

There are some methods on  global constants: 

class Card
  COLOURS = ['white', 'blue', 'black', 'red', 'green', 'yellow'].freeze
end

Loading a class variable: 

class Card
  def self.colours
    @colours ||= ['white', 'blue', 'black', 'red', 'green', 'yellow'].freeze
  end
end

Using top-level constant in config/initializers/my_constants.rb.

In Rails 5.0, configuration object directly for custom configuration

config/application.rb or config/custom.rb 

config.colours = %w(white blue black red green)

And creating in: 

Rails.configuration.colours # => ["white", "blue", "black", "red", "green"]

using  config.x  method: 

config.x.colours = %w(white blue black red green)

And available as:

Rails.configuration.x.colours # => ["white", "blue", "black", "red", "green"]