How to handle global variables in Ruby on Rails

Question

I just want to have an array as global, so when I add or remove an element, it can be reflected anywhere in the class.

For example:

class something
  @@my_array = Array.new
  def self.action_1
    @@my_array << 1
    @@my_array << 2
    @@my_array << 3
  end

   def self.how_many_elements
     puts "# of elements: " + @@my_array.size.to_s
   end
end

If i do the following:

something.action_1 => from controller_a

something.how_many_elements => from controller b

I always get the following output:

"# of elements: 0"

Why?

Quite aside from the fact that @@class variables won't do what you want here, it's never a good idea to use @@class variables in Ruby. Forget that they exist; use class @instance variables instead -- they function similarly but with fewer surprises. — Marnen Laibow-Koser
– Marnen Laibow-Koser, Commented Nov 29, 2011 at 17:46

tadman · Accepted Answer · 2011-11-29 17:30:42Z

5

It's a common mistake to think that you can stash things in your classes and they will persist between requests. If this happens it is purely coincidence and it is a behavior you cannot depend on.

Using global variables in this fashion is almost a bad idea. A properly structured Rails application should persist data in the session, the database, or the Rails.cache subsystem.

Each request serviced by Rails in development mode will start with a virtually clean slate, where all models, controllers, views and routes are reloaded from scratch each time. If you put things in a class thinking it will be there for you the next time around, you're going to be in for a surprise.

For saving things that are not important, use the Rails cache or the session facility. For saving things that are important, use a database.

answered Nov 29, 2011 at 17:30

tadman

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Comments

Geo · Accepted Answer · 2011-11-29 16:47:41Z

1

Use class variables:

class something

  @@my_array = []

  def self.action_1
    @@my_array << 1
    @@my_array << 2
    @@my_array << 3
  end

   def self.how_many_elements
     puts "# of elements: " + @@my_array.size
   end
end

answered Nov 29, 2011 at 16:47

Geo

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1 Comment

Dave Newton Over a year ago

Either would work under normal circumstances, although class variables would be preferable. Turns out it's unrelated to the type, though.

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