Where's the Best Place to Define a Constant in a Ruby on Rails Application

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.

Rails: Where to define constants with Classes?

I tried putting them in config/initializers, but they would get cached and not update even when I saved a new version of the class/file.

You could define them in a module. Create a file called global_constants.rb in your models directory and add constants there:

app/models/global_constants.rb

module GlobalConstants
  ALL_DEMOGRAPHICS = [Demographic::Gender, Demographic::Age]
end

Then you can use the constants like so:

GlobalConstants::ALL_DEMOGRAPHICS

If you keep the module in lib folder, then make sure you have added lib to autoload:

config/application.rb

config.autoload_paths << Rails.root.join('lib')

Store constants in Rails

Option 1 (class variables):

class Provider
  @@FACEBOOK = "facebook"
  @@TWITTER = "twitter"
end

@user.authentications.create(:uid => "12345", :provider => Provider.FACEBOOK)

Using class variables is strongly discouraged though in some cases. A better way to go would be to use meta voodoo:

class Provider
  @FACEBOOK = "facebook"
  @TWITTER = "twitter"

  class << self
    attr_reader :FACEBOOK, :TWITTER
  end
end

@user.authentications.create(:uid => "12345", :provider => Provider.FACEBOOK)

where to put configuration constant in rails

Let's make it clear what is the path you walk through to your app loads the initilization files. You must have in mind this :

When you run rails s (from your app/script/rails file) that runs rails server,

• your app boot.rb file (which mainly deals with gems loading) is required and then your console args (here the server alias s) requirement is passed to the rails railties/lib/rails/command.rb file (which is in the rails source code).

• There, your app config/application.rb is required. Then, the Server class is instantiated and you are thrown in railties/lib/rails/command/server.rb file. In fact, the Server class inhererits from Rack::Server, so a rails app is rack app ! So, you'll find a config.ru file in your rails app and this loads the config/environment.rb file.

The config/environment.rb file does two things

• requiring your app config/applcation.rb, that loads the proper rails framework.( For info, you'll find in your app application.rb the Application class that inherits from Rails::Application, which inherits itself from the Rails::Engine, which inherits from a Rails::Railtie class)

• calling the initialize! bang method, that loads the config/environment/*.rb files, and then processes all the initializers process (more precisely, the initializers methods are defined in the railties/lib/rails/engine.rb) file. So, now, taking that into account, you should be able to define properly your constants.

To better understand the intialization process, I recommand you this great railscast, which mainly inspired this answer.

Where would I store static/constant values in a rails application?

It depends on where you need to access them.

If you need to use them throughout your application, you can put them in environment.rb

# environment.rb
#
# other global config info
HELLO_EVERYONE = "hiz"

If you need to access them only inside a specific class, you can define them in that model.

class Test < ActiveRecord::Base
  HELLO_EVERYONE = "hiz"
end

EDIT

The second case (where the constant is defined in the Test class), can also be accessed outside of the Test class, only it needs to be referenced as Test::HELLO_EVERYONE.

This may be helpful in cases where you have a list of items relevant to the domain of that object (like a list of US states) that you might use in a view (e.g. select_tag :address, :state, options_for_select(Address::STATES)). Although I might consider wrapping this inside of a class method instead of exposing the internal structure of the class.

class Address< ActiveRecord::Base
  STATES = ["AL", "AK", "AZ", "AR", ...]

  def self.states
    STATES
  end
end

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