Constantize raises Uninitialized Constant Error

Question

I'm working on this pretty thorny problem and decided I might be able to dynamically create a class that inherits from StandardError by doing this:

something = "JustForBelow"
error_class = "#{something}Error".constantize
error_class = StandardError.new

But I'm getting a really weird error (in my opinion), which is:

Uninitialized constant JustForBelowError

Aren't I initializing it right there?

(Essentially) same error comes up when I try this:

StandardError.const_get "#{something}Error"
# => NameError: uninitialized constant StandardError::JustForBelowClass

This feels really weird, because a) these are super random names; there aren't any conflicts, and b) I'm pretty sure I've used constantize like in the first example before. Any ideas what's going wrong?

Answer 1

ActiveSupport's constantize method just looks up a constant. It's a fancier version of const_get that does nice things like traverse a nested module structure.

To create a new error, you'll want to do something like this:

2.0.0-p247 :013 > Object.const_set("MyNewError", Class.new(StandardError))
 => MyNewError
2.0.0-p247 :014 > MyNewError.ancestors
 => [MyNewError, StandardError, Exception, Object, Kernel, BasicObject]

At which point, you can do "MyNewError".constantize and get back that new class object.

Edit Also note that const_get in your second example error is looking inside the namespace it was called on. In that case, inside of StandardError's scope.

For instance, if you have a class structure like:

module A
  class B
    CONSTANT = "hello world"
  end
end

Then you could get at that with "A::B::CONSTANT".constantize, or by doing Object.const_get("A").const_get("B").const_get("CONSTANT"). Same thing, just ActiveSupport made is smoother to do.