How to find the method definition in Ruby

Andrei Kaleshka

19 Sep 2013

For the first glance it seems rather easy problem to find method definition. You know class name for the object, open documentation for this class and read it. But in real life it’s no so simple as seems. In Ruby the method for object can be defined from many places: modules, inheritance, meta-programming, other language’s extensions and etc. Imagine that you have installed a lot of gems in your application and every gem potentially can define or redefine method on any object. How to find the method definition in this situation?

Source location

Getting location of method definition is rather simple. Assume you have class A defined in the a.rb like this:

a.rb

class A
  def m
  end
end

In Ruby each method is an object too and you can get it using method with called #method. It returns object of class Method:

m = A.new.method(:m) #=> #<Method: A#m>

Object of class Method has ‘#source_location’ method which returns the file where the method defined and the line:

m.source_location #=> => ["/Users/ka8725/sl/a.rb", 2]

Now we know where the method is defined, open this file and see how it’s implemented. The method doesn’t work for methods which are defined with other languages extensions:

{}.method(:[]).source_location #=> nil

And pay attention that methods which are defined by meta-programming can lose their source location too:

b.rb

class B
  class_eval <<-M
    def m
    end
  M
end

B.new.method(:m).source_location #=> ["(eval)", 1]

Avoid missing line this way:

b.rb

class B
  class_eval <<-M, __FILE__, __LINE__
    def m
    end
  M
end

B.new.method(:m).source_location #=> ["/Users/ka8725/sl/activebilling/b.rb", 5]

Feel free to use #define_method - it doesn’t lose it’s source location. But if method appears on the object with #method_missing help it can’t know anything about it’s definition place.

Practical application

Recently I had a problem in our Rails application. I defined Resource model ResourcesController and as usual it’s routes but there was surprise when I opened page which contained this helper: link_to 'Resource', resource_path(@resource). This code caused exception SystemStackError - stack level too deep. Rather strange situation, isn’t it? Going through sources of Rails I didn’t find any collisions there but finally with Method#source_location help I found the source of this method definition (it’s inherited_resources gem which we use) and as a result I patched this gem. I would say that patch is good but at least it looks logical.

Conclusion

No doubt that Method#source_location method is very helpful. But sometimes it can be useless whatever. So the conclusion here is avoid using #method_missing (there are many reasons to avoid it and this is one of the all cases).