Ruby on Rails: Where to define global constants?

Question

I\'m just getting started with my first Ruby on Rails webapp. I\'ve got a bunch of different models, views, controllers, and so on.

I\'m wanting to find a good plac

Answer 1

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.

Answer 2

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

Answer 3

The global variable should be declare in config/initializers directory

COLOURS = %w(white blue black red green)

Answer 4

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.

Answer 5

If a constant is needed in more than one class, I put it in config/initializers/contant.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

Answer 6

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');