Question

Whenever I talk to Rubyists I hear great things about method_added and method_missing. However, I got recently chided for using both in a project. The argument was that another library (such as rspec, for example) could overload the methods too and put the program in a strange state depending on which version of method_missing got called first.

I’m curious to know how often this happens. Is it really that dangerous to overload method_missing? Does anyone have real-world examples of woe arising from method_missing conflicts?

Answer 1

One thing to keep in mind that if you (re)define method_missing on a class, you’re replacing any previous implementations of this method (from a Gem for example).
You can avoid this by creating a new class that inherits from the class you want to extend with method_missing.

However, this is normally not an issue as most Gems have their own classes.

The other thing to keep in mind is to always call super at the end of method_missing to not break the method invocation chain.

def method_missing(m, *args, &block)
  # your code...

  super
end

Maybe this Graphic of the Ruby Method Lookup Flow is also helpful.