Overriding Class Methods With Modules

[John W. Long]

How do I override a class method on an object with another module? For
example:

class TestObject
def self.my_method
"override me"
end
end

module TestExtension
def my_method
"overridden by TestExtension"
end
def another_method
"another method"
end
end

TestObject.extend TestExtension

puts TestObject.my_method #=> "override me"
puts TestObject.another_method #=> "another method"

Why can't I get TestObject to use my_method from TestExtension?

 

[Trans]

How do I override a class method on an object with another module? For
example:

class TestObject
def self.my_method
"override me"
end
end

module TestExtension
def my_method
"overridden by TestExtension"
end
def another_method
"another method"
end
end

TestObject.extend TestExtension

puts TestObject.my_method #=> "override me"
puts TestObject.another_method #=> "another method"

Why can't I get TestObject to use my_method from TestExtension?

Because we don't have Cuts ;-)

Okay a more presently practical answer. You can use a hack like:

module TestExtension
def self.included(base)
base.module_eval do
def my_method
"overridden by TestExtension"
end
end
end
def another_method
"another method"
end
end

T.

 

[John W. Long]

Okay a more presently practical answer. You can use a hack like:

module TestExtension
def self.included(base)
base.module_eval do
def my_method
"overridden by TestExtension"
end
end
end
def another_method
"another method"
end
end

Mmm. That doesn't work either.

 

[Jan Svitok]

shouldn't be "extended" ?

Mmm. That doesn't work either.

 

[dblack]

Hi --

How do I override a class method on an object with another module? For
example:

class TestObject
def self.my_method
"override me"
end
end

module TestExtension
def my_method
"overridden by TestExtension"
end
def another_method
"another method"
end
end

TestObject.extend TestExtension

puts TestObject.my_method #=> "override me"
puts TestObject.another_method #=> "another method"

Why can't I get TestObject to use my_method from TestExtension?

What you're seeing is that in the method lookup path, a given class
comes before any modules it includes. In your case, the class in
question is the singleton class of TestObject: that class defines
#my_method, and also includes a module that defines #my_method. The
one defined in the class "wins".

Here's another, parallel example:

module M
def my_method
puts "M#my_method"
end
end

class C
end

c = C.new

class << c # defined the method in the class
def my_method
puts "C#my_method"
end
end

c.extend(M) # also include the module in the class
c.my_method # "C#my_method" (the class wins)

The module would win if the method were defined in the *class* of the
object, rather than the singleton class -- because then the lookup
order would be:

singleton class (no)
module included by singleton class (yes -- execute method)
[class (never reached)]


David

--
David A. Black | (e-mail address removed)
Author of "Ruby for Rails" [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB's Weblog) [2] | Co-director, Ruby Central, Inc. [4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com | [4] http://www.rubycentral.org

 

[Aleks Kissinger]

That works. So the final code would be something like:

module TestExtension
def inject_methods(base)
base.module_eval do
def self.my_method
"overridden by TestExtension"
end
end
end

def extended(base)
inject_methods(base)
end

def included(base)
inject_methods(base)
end
end

This has always been a little clunky in Ruby, primarily because
methods written within a specific class were designed to override
mixin and parent class methods, not vice-versa.