How to set constant dynamically in Ruby on Rails helper function?

Question

I have this (simplified) helper function in Rails:

include Constants

def link_to_neighbour(direction, path)
  symbol = direction.upcase.constantize
  link_to symbol, path
end

In lib/constants I defined these constants:

PREVIOUS = "<<"
NEXT = ">>"

Yet when I use something like this in one of my views...

<%= link_to_neighbour('next', @user, user_path(@user)) %>

... I constantly get this error:

NameError
uninitialized constant NEXT

What am I missing here?

Thanks for any help.

I am not sure about the solultion to your problem, but why are you using so complicated solution just to print two chars? Isn't it better to go for translations maybe? — Mike Szyndel
– Mike Szyndel, Commented Jul 16, 2013 at 11:44
The problem occurs probably because your constants are namespaced, so you should refer them as Constants::PREVIOUS and Constants::NEXT. — Marek Lipka
– Marek Lipka, Commented Jul 16, 2013 at 11:50
I tried that already. I am using other constants in that same file too and never needed the Constants:: prefix there. — Tintin81
– Tintin81, Commented Jul 16, 2013 at 12:30

the Tin Man · Accepted Answer · 2014-10-07 21:58:34Z

2

You can use const_missing hook of ruby.

def self.const_missing(name)
  const_set(name, some_value)
end

but the problem here seems that you have not loaded 'lib/constants.rb' file in application.rb of your application.

Put this line in your 'config/application.rb'

# Autoload lib/ folder including all subdirectories
config.autoload_paths += Dir["#{config.root}/lib/**/"]

If you don't want to put this line in your 'config/application.rb' then move constants.rb from lib to your 'config/initializers/' folder. Your Rails application loads each file there automatically.

edited Oct 7, 2014 at 21:58

the Tin Man

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answered Jul 16, 2013 at 12:06

Sachin Singh

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Comments

the Tin Man · Accepted Answer · 2014-10-07 21:58:45Z

1

Your method should look like this:

def link_to_neighbour(direction, path)
  symbol = Object.const_get(direction.upcase)
  link_to symbol, path
end

Hope this will help.

edited Oct 7, 2014 at 21:58

the Tin Man

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answered Jul 16, 2013 at 12:11

Rails Guy

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Comments

barvaz · Accepted Answer · 2013-07-16 22:28:58Z

0

I'd use a constants.yml

directions:
  PREVIOUS: "<<"
  NEXT: ">>"

this way I can use a tree of constants.

then in an initializer:

Constants = OpenStruct.new YAML.load_file(Rails.root.join('config/constants.yml')).with_indifferent_access

then in the helper method:

def link_to_neighbour(direction, path)
  symbol = Constants.directions[direction.upcase]
  link_to symbol, path
end

answered Jul 16, 2013 at 22:28

barvaz

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