How do I get class-object from string “A::B::C” in Ruby?

Question

The following example fails

class A
  class B
  end
end
p Object.const_get \'A\' # => A
p Object.const_get \'A::B\' # => NameError: wrong constant name A::         


        

Answer 1

You'll have to manually "parse" the colons yourself and call const_get on the parent module/class:

ruby-1.9.1-p378 > class A
ruby-1.9.1-p378 ?>  class B
ruby-1.9.1-p378 ?>    end
ruby-1.9.1-p378 ?>  end
 => nil 
ruby-1.9.1-p378 > A.const_get 'B'
 => A::B 

Someone has written a qualified_const_get that may be of interest.

Answer 2

Extlib provides a full_const_get method, which does just this.

http://github.com/datamapper/extlib/blob/master/lib/extlib/object.rb#L67

You could either include extlib, or copy this implementation and use it yourself (assuming the licensing is compatible with whatever you're using it for)

Answer 3

Here is Rails' constantize method:

def constantize(camel_cased_word)
  names = camel_cased_word.split('::')
  names.shift if names.empty? || names.first.empty?

  constant = Object
  names.each do |name|
    constant = constant.const_defined?(name) ? constant.const_get(name) : constant.const_missing(name)
  end
  constant
end

See, it starts at the Object on top of it all, then uses each name in between the double semicolons as a stepping stone to get to the constant you want.