warning: toplevel constant referenced

Question

I have four models (Document, Question, Question::Document, and Answer). In my Answer model I have

validates :text,
  presence: { :unless => Proc.new{ |a| a.question.is_a? Question::Document } }

This gives me the the warning

warning: toplevel constant Document referenced by Question::Document

How do I prevent this warning from happening (without renaming my classes)?

Answer 1

Your folder/file structure should look as follows:

app/
  models/
    question/
      document.rb
    answer.rb
    document.rb
    question.rb

And then rails will automatically find the correct models (it will translate the model name to a filename, and namespaces are translated to folders).

Make sure that inside your question/document.rb the class definition looks as one of the following alternatives:

class Question::Document
end

or

class Question
  class Document
  end
end

If you write just class Document you are redefining the toplevel constant Document.

Note that if the global Document is defined first, this will also trigger this error. This depends on the code path, so the best way to resolve that, is to add a require_dependency where needed. See here and here for more background.

E.g. something like

require_dependency 'question/document' 

class Answer < ActiveRecord::Base

end  

If you put the file in a different place, where rails cannot automatically find it, you will have to explicitly require it, so rails knows Question::Document exists.

If for instance, you define Question::Document inside the Question model, which is a reasonable place, you will have to explicitly state the dependency to the Question model in your Answer model.

So, in that case, in your answer.rb you will write

require_dependency 'question'

class Answer < ActiveRecord::Base
  # ..
end

While plain require works, it is preferred to use require_dependency instead as it will work with auto-loading, which means: behaves as expected during development.

Answer 2

In Rails, you are not supposed to use "require" as it messes up autoloading.

One solution to this is to append a require_dependency to the end of the file that defines the outer constant.

app/models/question.rb

class Question
  # ...
end

require_dependency 'question/document'

app/models/question/document.rb

class Question
  class Document
    # ...
  end
end

This forces the Question::Document class to be loaded after the Question constant is found. Normally, if Rails already knows about the top-level Document constant, it won't attempt to load Question::Document if it's not already known.

References:

• This example in the Rails Guides on Autoloading and Reloading Constants
• require_dependency at ApiDock

Answer 3

You need to have Question::Document defined before referencing offending Document reference. Otherwise Ruby will start traversing namespaces up to find Document. Your answer.rb should have

require 'question/document'

on top of it, assuming that's the path where Question::Document is defined.

Answer 4

You might be seeing the warning like this

/path/to/app/models/answer.rb:4: warning: toplevel constant Document referenced by Question::Document

Just require the class which was referenced , on the top file which is throwing this warning.

In your case the below line will go in app/model/answer.rb

require Rails.root.join('app/models/question/document.rb')

And, after restarting the rails server you won't see such ugly warning.

You can read detail explanation here

Answer 5

Put the various class definitions in order so that Question::Document is defined before you reference it. Otherwise ruby goes looking in the toplevel, as 7stud showed.

test.rb

class Document
end

class Question
end

class Question
  class Document
  end
end

class Answer
  puts Question::Document.class
end

The result

$ ruby test.rb
Class

Answer 6

I've wrote a gem which introduce alternative to require_dependency solution: heavy_control

It explicitly resolves given constant names on initalization via constantize (before other constants are loaded). Also it happens each reload in development.

Answer 7

You mean like this:

Document = 'hello'

class Question
end

class Animal
  puts Question::Document
end

class Question
  class Document
  end
end

--output:--
1.rb:7: warning: toplevel constant Document referenced by Question::Document
hello

It looks like ruby searches enclosing scopes for a constant when it isn't found in the specified scope. I think the warning is a penalty for being unable to think up more than two variable names.