Question

Below I created a class method in a model called Movie that should return an array:

def self.all_ratings
  Array['G','PG','PG-13','R','NC-17']
end

And in my movies controller I access it using the following instance variable:

@all_ratings = Movie.all_ratings

However when it comes time to use it in my index view I receive the following errors:

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.each

I believe I am creating the array properly but I could be wrong. Any suggestions why these errors occur?

Below is the view where @all_ratings is used:

%h1 All Movies

= form_tag movies_path, :method => :get do
  Include: 
  - @all_ratings.each do |rating|
    = rating
    = check_box_tag "ratings[#{rating}]"
  = submit_tag 'Refresh'

And here is how I implemented @all_ratings into the controller

class MoviesController < ApplicationController

  @all_ratings = Movie.all_ratings

Answer 1

The code to initialize an instance variable needs to be in an instance method.

(Otherwise the scope is the class, not an instance.)

class MoviesController < ApplicationController
  def index # Or wherever
    @all_ratings = Movie.all_ratings
  end
end

If you need that value in several methods, you could use, say, a before_filter.

Ruby is different from languages like, say, Java. In Java, instance variables are defined outside instance methods, and are available in every instance method, with whatever value they were initialized with.

In Ruby, there are two (major) ways to handle instance variables: use a method like attr_accessor to create accessor methods, or initialize them inside an instance method, as shown above.

Once an instance variable has initialized, its value is usable from any other instance method. For example, in your comments you mention initializing it in a ratings method. Unless ratings is explicitly called, @all_ratings will not be initialized. In other words, if you make a GET request to index, the ratings method will not be called, and @all_ratings will still be nil.

If you explicitly call ratings from index, then @all_ratings will be initialized (by the ratings method). Once it’s initialized in any instance method, that instance of the object (the controller in this case) has an initialized @all_ratings instance variable:

def index
  ratings
end

Now the value of @all_ratings is available in index’s template.

Without putting the instance variable initialization in an instance method what you’re actually doing is creating an instance variable in the class Foo, which is something very different:

[1] pry(main)> class Foo
[1] pry(main)*   @wat = self.class
[1] pry(main)* end  
=> Class
[2] pry(main)> f = Foo.new
=> #<Foo:0x007fbfba8efba8>
[5] pry(main)> f.instance_variables
=> []
[6] pry(main)> Foo.instance_variables
=> [:@wat]
[7] pry(main)> Foo.instance_eval "@wat"
=> Class