Ruby on Rails: Where to define global constants?

If your model is really "responsible" for the constants you should stick them there. You can create class methods to access them without creating a new object instance:

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

# accessible like this
Card.colours

Alternatively, you can create class variables and an accessor. This is however discouraged as class variables might act kind of surprising with inheritance and in multi-thread environments.

class Card < ActiveRecord::Base
  @@colours = ['white', 'blue'].freeze
  cattr_reader :colours
end

# accessible the same as above
Card.colours

The two options above allow you to change the returned array on each invocation of the accessor method if required. If you have true a truly unchangeable constant, you can also define it on the model class:

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

# accessible as
Card::COLOURS

You could also create global constants which are accessible from everywhere in an initializer like in the following example. This is probably the best place, if your colours are really global and used in more than one model context.

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

# accessible as a top-level constant this time
COLOURS

Note: when we define constants above, often we want to freeze the array. That prevents other code from later (inadvertently) modifying the array by e.g. adding a new element. Once an object is frozen, it can't be changed anymore.

Some options:

Using a constant:

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

Lazy loaded using class instance variable:

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

If it is a truly global constant (avoid global constants of this nature, though), you could also consider putting a top-level constant in config/initializers/my_constants.rb for example.

As of Rails 4.2, you can use the config.x property:

# config/application.rb (or config/custom.rb if you prefer)
config.x.colours.options = %w[white blue black red green]
config.x.colours.default = 'white'

Which will be available as:

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

Another method of loading custom config:

# config/colours.yml
default: &default
  options:
    - white
    - blue
    - black
    - red
    - green
  default: white
development:
  *default
production:
  *default

# config/application.rb
config.colours = config_for(:colours)

Rails.configuration.colours
# => {"options"=>["white", "blue", "black", "red", "green"], "default"=>"white"}
Rails.configuration.colours['default']
# => "white"

In Rails 5 & 6, you can use the configuration object directly for custom configuration, in addition to config.x. However, it can only be used for non-nested configuration:

# config/application.rb
config.colours = %w[white blue black red green]

It will be available as:

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

If a constant is needed in more than one class, I put it in config/initializers/constant.rb always in all caps (list of states below is truncated).

STATES = ['AK', 'AL', ... 'WI', 'WV', 'WY']

They are available through out the application except in model code as such:

    <%= form.label :states, %>
    <%= form.select :states, STATES, {} %>

To use the constant in a model, use attr_accessor to make the constant available.

class Customer < ActiveRecord::Base
    attr_accessor :STATES

    validates :state, inclusion: {in: STATES, message: "-- choose a State from the drop down list."}
end

For application-wide settings and for global constants I recommend to use Settingslogic. This settings are stored in YML file and can be accessed from models, views and controllers. Furthermore, you can create different settings for all your environments:

  # app/config/application.yml
  defaults: &defaults
    cool:
      sweet: nested settings
    neat_setting: 24
    awesome_setting: <%= "Did you know 5 + 5 = #{5 + 5}?" %>

    colors: "white blue black red green"

  development:
    <<: *defaults
    neat_setting: 800

  test:
    <<: *defaults

  production:
    <<: *defaults

Somewhere in the view (I prefer helper methods for such kind of stuff) or in a model you can get, for ex., array of colors Settings.colors.split(/\s/). It's very flexible. And you don't need to invent a bike.

Try to keep all constant at one place, In my application I have created constants folder inside initializers as follows:

https://i.stack.imgur.com/UODRT.png

and I usually keep all constant in these files.

In your case you can create file under constants folder as colors_constant.rb

colors_constant.rb

https://i.stack.imgur.com/2X2Ny.png

Don't forgot to restart server

Use a class method:

def self.colours
  ['white', 'red', 'black']
end

Then Model.colours will return that array. Alternatively, create an initializer and wrap the constants in a module to avoid namespace conflicts.

Another option, if you want to define your constants in one place:

module DSL
  module Constants
    MY_CONSTANT = 1
  end
end

But still make them globally visible without having to access them in fully qualified way:

DSL::Constants::MY_CONSTANT # => 1
MY_CONSTANT # => NameError: uninitialized constant MY_CONSTANT
Object.instance_eval { include DSL::Constants }
MY_CONSTANT # => 1

A common place to put application-wide global constants is inside config/application.

module MyApp
  FOO ||= ENV.fetch('FOO', nil)
  BAR ||= %w(one two three)

  class Application < Rails::Application
    config.foo_bar = :baz
  end
end

I typically have a 'lookup' model/table in my rails program and use it for the constants. It is very useful if the constants are going to be different for different environments. In addition, if you have a plan to extend them, say you want to add 'yellow' on a later date, you could simply add a new row to the lookup table and be done with it.

If you give the admin permissions to modify this table, they will not come to you for maintenance. :) DRY.

Here is how my migration code looks like:

class CreateLookups < ActiveRecord::Migration
  def change
    create_table :lookups do |t|
      t.string :group_key
      t.string :lookup_key
      t.string :lookup_value
      t.timestamps
    end
  end
end

I use seeds.rb to pre-populate it.

Lookup.find_or_create_by_group_key_and_lookup_key_and_lookup_value!(group_key: 'development_COLORS', lookup_key: 'color1', lookup_value: 'red');

The global variable should be declare in config/initializers directory

COLOURS = %w(white blue black red green)

According your condition, you can also define some environmental variables, and fetch it via ENV['some-var'] in ruby code, this solution may not fit for you, but I hope it may help others.

Example: you can create different files .development_env, .production_env, .test_env and load it according your application environments, check this gen dotenv-rails which automate this for your.

I think you can use gem config

https://github.com/rubyconfig/config

Easy handle and edit