What is the reason to use ||= instead of = in model to define a constant

Question

Found this in a current project:

class Video < ActiveRecord::Base
  ...
  VALID_VIDEO_HOSTS ||= %w(www.youtube.com youtube.com vimeo.com www.vimeo.com)
  ...
end

Usually I use simple VALID_VIDEO_HOSTS = %w(...) and haven't any guess why the previous developers used ||=

Anybody know what are the benefits?

UPDATE

I know what does ||= in general cases, the question is mostly about defining constant that way in the model.

I've not seen that with a constant before. Generally, the pattern for constants is `unless defined? CONSTANT_NAME`, as this answer shows http://stackoverflow.com/a/10172072/335847. — iain, Apr 17 '13 at 21:44

score 4 · Accepted Answer · answered Apr 17 '13 at 21:49

||= is used as a cheap way to memoize the value, as the other posters mention. However...

Why memoize a constant?

The author is most likely protecting against warnings when loading that source file multiple times. (warning: already initialized constant VALID_VIDEO_HOSTS)

Tilo · Answer 2 · 2013-04-17T22:07:27.643

||= memoizes the value -- it sets the value only once -- only if it's not previously defined. When called subsequently, the value is already set and will not be modified. It uses Ruby's lazy loading to evaluate if the left-hand side as a boolean value, and only does the assignment if it is nil or false.

These two lines are equivalent:

  var ||= something

  var = something if var.nil?   # this is equivalent to ||=

  # Note: memoizing with ||= only works if the right-hand side is not a boolean function;
  #       because if it's value would be false, it could not use Ruby's lazy-loading

This is often used for memoizing (caching) if the right-hand-side is an expensive / time-consuming operation, or in general during initializations which have to be done only once.

Why ||= when assigning to a Constant?

Constants should only initialized once, and will at least issue a warning when you try to override them. The ||= makes sure the value is assigned only once to the constant. This way you don't get a warning that the constant is already initialized.

they are **not** identical. think about case when `var == false` — David Unric, Apr 17 '13 at 21:47
that's true. You can not memoize a boolean function with ||= :) — Tilo, Apr 17 '13 at 21:51
You can, just not with that syntax. `if defined? val` can be used to memoize falsy values — Nevir, Apr 17 '13 at 21:52

score 1 · Answer 3 · answered Apr 17 '13 at 21:47

1

That call that a memoize - means you set it once, and then keep that value, like a lazy load of the value.

ActiveSupport has a method for this now that works slightly differently than ||=: http://apidock.com/rails/ActiveSupport/memoize

answered Apr 17 '13 at 21:47

Andrew Kuklewicz

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score 0 · Answer 4 · answered Apr 17 '13 at 21:47

It's used for lazily initializating a value. In your example there's not much call for it because the overhead of creating the string array is not that much. A better example would be:

class WebsiteImageHelper
    def get_image_files
      #recurse directories looking for images
      #this will only happen once per instance of 
      #WebsiteImageHelper
      @image_files ||= Dir['images/**/**']
    end
end

score 0 · Answer 5 · answered May 01 '15 at 21:26

0

I personally use it in a project to allow user to load code. User code may override that constant if user desires. So I avoid error.

answered May 01 '15 at 21:26

akostadinov

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What is the reason to use ||= instead of = in model to define a constant

5 Answers5