How to rescue the error exception raised by the `constantize` method?

Question

I am using Ruby on Rails 3.2.2 and I would like to properly rescue the following process flow by raising a "custom" error message:

def rescue_method
  # sample_string.class
  # => String
  # sample_string.inspect
  # => "ARubyConstantThatDoesNotExist"

  begin
    build_constant(sample_string)
  rescue
    raise("My custom error message: #{build_constant(sample_string)} doesn't exist.")
  end
end

def build_constant(sample_string)
  "AModule::#{sample_string}".constantize
end

Note: I feel "forced" to use the constantize method also in the raised "custom" message in order to DRY code...

When the rescue_method is executed it seems that the raise("My custom error message") code is never executed and I get the following error:

uninitialized constant AModule::ARubyConstantThatDoesNotExist

How to properly display the raised "custom" message (since a further error exception is raised in the subsequent raised "custom" message)? What do you advice about?

Answer 1

The problem is that your build_constant method is doing two things:

1. Building the class name.
2. Turning the name into a class using constantize.

One of those things wants to use the other when an exception is raised. A simple solution is to pull those separate tasks apart:

def build_class_name(sample_string)
  "AModule::#{sample_string}"
end

def rescue_method
  name = build_class_name(...)
  name.constantize
rescue NameError
  raise("My custom error message: #{name} doesn't exist.")
end

You should also be more specific about the exception you're looking for so I added that for free.

Answer 2

If you don't want to rely on catching any exception, you could use safe_constantize (https://apidock.com/rails/ActiveSupport/Inflector/safe_constantize).

Same purpose as constantize but will return nil instead, whenever a Module does not exist.

'UnknownModule::Foo::Bar'.safe_constantize  # => nil

Answer 3

begin
  "AModule::#{sample_string}".constantize
rescue SyntaxError, NameError => err
  raise("My custom error message")
end