CSC/ECE 517 Fall 2011/ch3 3i ws
Introduction
Method missing is very powerful and widely used in Ruby on Rails. As one of the dynamic features of Ruby, it is a way of intercepting calls to methods that haven't been defined, which would otherwise raise a NoMethodError. The functionality of method missing is one of the core ingredients of cleanly designed Ruby programs.<ref>Method Missing http://contextr.rubyforge.org/test/method_missing.html</ref> It is still possible to extend it with context-dependent behavior.
This feature is not unique to Ruby. It also exists in Smalltalk, Python, Groovy, some JavaScripts, and even most CLOS extensions have it.
Why it is used
When we send a message to an object, the object executes the first method it finds on its method lookup path with the same name as the message. If it fails to find any such method, it raises a NoMethodError exception - unless you have provided the object with the method_missing.<ref>Ruby Method Missing http://rubylearning.com/satishtalim/ruby_method_missing.html</ref> The method_missing method is passed the symbol of the non-existent method, an array of the arguments that were passed in the original call and any block passed to the original method.
How it is used
When we call a method that doesn't exist, Ruby looks for it in the receiving object's class first. If it doesn't find the method there, it looks for it in the superclass, and then the superclass and the superclass, and so on, until it eventually reaches the top of the chain (Object in Ruby 1.8, BasicObject in Ruby 1.9).<ref>[3] Method missing in Ruby. Does it make execution of functions in a Rails app any faster. http://www.coderanch.com/t/489410/Ruby/Method-missing-Ruby-Does-it</ref> Only then, if it still didn't find the method, does Ruby get down in the receiver's class again to look for a method_missing. So, method_missing is generally slower than a regular method.
Adding better debug information on failure
One of the most simple but still very powerful ways of using method_missing is to allow it to include more information in the error message.<ref>Patterns of method missing http://ruby.dzone.com/news/patterns-method-missing</ref> Here is a simple example:
class MyFoo
def method_missing(method, *args, &block)
raise NoMethodError, <<ERRORINFO
method: #{method}
args: #{args.inspect}
on: #{self.to_yaml}
ERRORINFO
end
end
Other patterns of method missing
There are many other patterns of method_missing. For example, using Ruby’s blocks and method_missing to make it easy to create any kind of output structure, like XML, HTML, graphical UIs and other hierarchical data structures, to lend themselves very well to the builder pattern. By using method_missing, all kinds of test helpers, which can be used to implement factories, delegate and do all kinds of things, are able to be created.
Examples
Roman Numerals
The classic example for missing_method, given in the Ruby Docs:<ref>Ruby Docs: Object.missing_method http://ruby-doc.org/docs/ProgrammingRuby/html/ref_c_object.html#Object.method_missing </ref>
class Roman
def romanToInt(str)
# ...
end
def method_missing(methId)
str = methId.id2name
romanToInt(str)
end
end
Usage:
r = Roman.new r.iv » 4 r.xxiii » 23 r.mm » 2000
The Roman class has no .iv or .xxiii methods, but the method_missing method turns the message into a string with id2name (which takes symbols and returns them as strings), and passes it to the romanToInt. This example shows how method_missing can make the Ruby language very flexible.
method_missing in Rails
Rails, like many domain specific languages of Ruby, makes heavy use of the method_missing function. A prominent example is ActiveRecord, where dynamic finders are implemented using method_missing. When a call like find_by_username is called from an ActiveRecord object, the method does not explicitly exist, rather the message is picked apart in ActiveRecord's Base::method_missing method. The variable part of the message, in this case "username" is turned into an argument to the find method, which is defined. The code below is not exactly what is in the method itself, but outlines what actually happens after calls to helper other methods:<ref>How dynamic finders work http://blog.hasmanythrough.com/2006/8/13/how-dynamic-finders-work </ref>
def method_missing(method_id, *arguments)
if match = /find_(all_by|by)_([_a-zA-Z]\w*)/.match(method_id.to_s)
# find...
elsif match = /find_or_create_by_([_a-zA-Z]\w*)/.match(method_id.to_s)
# find_or_create...
else
super
end
end
Considerations
Drawbacks
All else being equal, method_missing is less efficient at runtime than normally defined methods. Besides the fact that the missing_method will need additional checks to determine the next course of action, method_missing is always the last place the ruby interpreter looks. <ref> method_missing: price to pay you http://www.alef1.org/ruby/method_missing/ </ref>
Method_missing can make the code more difficult for humans to follow. It is also does not work well with autocomplete and other features of IDE's or editors.
All of these things should be considered when deciding whether to rely on method_missing.
Relationship to respond_to?
Ruby objects also have a method called respond_to? which checks to see that a given message corresponds to a method of the class. By default, it will only look for explicitly defined methods. If none are found, it has no mechanism to check whether it is handled by method_missing. This issue is important to consider if the class or its children ever need the respond_to? method. It is good practice to always override the respond_to? method when using method_missing in a class that might be subclassed. In the stub below, we add a respond_to to the Roman Numerals class.
class Roman
def respond_to?
if method_sym.to_s =~ ... // regex to see if it's a valid roman numeral
true
else
super
end
end
end
Similar functionality in other languages
The method_missing class is defined in the Object class, so it is inherited by all objects. It raises a NoMethodError by default.
There are no similar methods in statically typed languages like Java or C++. Python does not have an equivalent method either, though it can be simulated using the __getattr__ method <ref>Various imlementations in other languages http://langexplr.blogspot.com/2008/02/handling-call-to-missing-method-in_06.html </ref>. Firefox's JavaScript implementation now supports a similar __noSuchMethod__ object <ref>Javascript documentation https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/noSuchMethod</ref> method, but it is not a JavaScript standard.
References
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