Cast between String and Classname

Question

I have a string, containing an Class name. It is, for example, a string containing "Article". That string came up from the params[]. What should I do to work with this string as if it was a class name? For instance, I want to do:

Article.all

and so on.

Any idea?

Answer 1

class Abc
end #=> nil
klass = eval("Abc") #=> Abc
klass.new #=> #<Abc:0x37643e8>

Assumes there really is a class with the name provided...

In ActiveSupport, there was String#constantize, which did the same thing, but I believe it's deprecated after 2.1.

EDIT: this is the implementation of constantize from ActiveSupport 2.1.2:

  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

Answer 2

This solution is better than eval as you are evaluating params hash that might be manipulated by the user and could contain harmful actions. As a general rule: Never evaluate user input directly, that's a big security hole.

# Monkey patch for String class
    class String
      def to_class
        klass = Kernel.const_get(self)
        klass.is_a?(Class) ? klass : nil
      rescue NameError
        nil
      end
    end

# Examples
"Fixnum".to_class #=> Fixnum
"Something".to_class #=> nil

Update - a better version that works with namespaces:

 # Monkey patch for String class
    class String
      def to_class
        chain = self.split "::"
        klass = Kernel
        chain.each do |klass_string|
          klass = klass.const_get klass_string
        end
        klass.is_a?(Class) ? klass : nil
      rescue NameError
        nil
      end
    end

Answer 3

I am not sure whether I understand your intention correctly. Here I assume all is an Class method of Article and all return an array of articles.

class Article
    def self.all
       ["Peopleware" , "The Mythical Man-Month"]
    end

end

s = "Article"
all_of_article = []
eval("all_of_article = #{s + ".all"}")
puts all_of_article.inspect  # ["Peopleware", "The Mythical Man-Month"]