# Alias new?

**URL:** <https://www.ruby-forum.com/t/alias-new/98045>\
**Category:** Ruby\
**Created:** [July 12, 2006, 6:10am UTC](https://www.ruby-forum.com/t/alias-new/98045 "2006-07-12T06:10:09Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![crucoba](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e8c25b/32.png) [@crucoba](https://www.ruby-forum.com/u/crucoba)\
**Post date:** [July 12, 2006, 6:10am UTC](https://www.ruby-forum.com/t/alias-new/98045/1 "2006-07-12T06:10:09Z")

</div>

I’ve seen this work before:

module MagicWithMethodMissing  
alias\_method :\_\_orig\_method\_missing, :method\_missing  
def method\_missing(sym, \*args, &block)  
if some\_condition  
#do special stuff  
else  
\_\_orig\_method\_missing(sym, \*args, &block)  
end  
end  
end

class MyClass  
include MagicWithMethodMissing  
end

I want to do the same thing w/ :new on the class

module MagicWithNew  
alias\_method :\_\_orig\_new, :new  
def new(\*args, &block)  
if some\_condition  
#do special stuff  
else  
\_\_orig\_new(\*args, &block)  
end  
end  
end

The question I have is how do I get this included in the meta class  
of MyClass?

Thanks,  
David

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<div class="post-metadata">

**Author:** ![crucoba](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e8c25b/32.png) [@crucoba](https://www.ruby-forum.com/u/crucoba)\
**Post date:** [July 12, 2006, 6:50am UTC](https://www.ruby-forum.com/t/alias-new/98045/2 "2006-07-12T06:50:03Z")

</div>

On Jul 12, 2006, at 12:07 AM, David C. wrote:

> end  
> def new(\*args, &block)
> 
> Thanks,  
> David

Couple of things

1. alias\_method gets executed at “compile” time. This means that it  
will try to alias MagicWithNew.

2. This is ok because you dont want to use alias’s in this case.  
Since #new is almost never overriden (it’s usually #initialize.  
Incidentally are you sure you don’t want to define #initialize  
instead?) it’s pretty much guaranteed that you class is using the  
original implementation in Class.  
so we can use inheritance and super

module MagicWithNew  
def new(\*args, &block)  
if some\_condition  
do\_special\_stuff  
else  
super  
end  
end  
end

class MyClass  
extend MagicWithNew # note #extend, not #include  
end

---

<div class="post-metadata">

**Author:** ![crucoba](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e8c25b/32.png) [@crucoba](https://www.ruby-forum.com/u/crucoba)\
**Post date:** [July 12, 2006, 7:02am UTC](https://www.ruby-forum.com/t/alias-new/98045/3 "2006-07-12T07:02:37Z")

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On Wed, 12 Jul 2006, Logan C. wrote:

> module MagicWithNew  
> def new(\*args, &block)  
> if some\_condition  
> do\_special\_stuff

fails to initialize object! you need the old ‘new’ here. or super.

> else  
> super  
> end  
> end  
> end
> 
> class MyClass  
> extend MagicWithNew # note #extend, not #include  
> end

here is an example of a fairly robust pattern

harp:~ \> cat a.rb  
module M  
ClassMethods = lambda do  
alias\_method ‘ **new** ’, ‘new’ # correct alias of new

```
   def new *a, &b
     obj = __new__ (*a, &b)
   #
   # do anything you want with obj here based on any condition. 

```

this is just  
# an example.  
#  
class \<\< obj  
def answer() 42 end  
end  
obj  
end  
end

```
 def self.included other
   meta =
     class << other
       self
     end
   meta.module_eval &ClassMethods
   super
 end

```

end

class C  
include M  
end

p C.new.answer

harp:~ \> ruby a.rb  
42

regards.

-a

---

<div class="post-metadata">

**Author:** ![crucoba](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e8c25b/32.png) [@crucoba](https://www.ruby-forum.com/u/crucoba)\
**Post date:** [July 12, 2006, 7:21am UTC](https://www.ruby-forum.com/t/alias-new/98045/4 "2006-07-12T07:21:23Z")

</div>

On Jul 12, 2006, at 1:01 AM, removed\_email\_address@domain.invalid wrote:

> > original implementation in Class.  
> > so we can use inheritance and super
> > 
> > module MagicWithNew  
> > def new(\*args, &block)  
> > if some\_condition  
> > do\_special\_stuff
> 
> fails to initialize object! you need the old ‘new’ here. or super.

Well he did say “special stuff”. He only needs old new or super there  
if he plans on initializing the object 😉

---

<div class="post-metadata">

**Author:** ![crucoba](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e8c25b/32.png) [@crucoba](https://www.ruby-forum.com/u/crucoba)\
**Post date:** [July 12, 2006, 12:59pm UTC](https://www.ruby-forum.com/t/alias-new/98045/5 "2006-07-12T12:59:06Z")

</div>

On Jul 12, 2006, at 12:15 AM, Logan C. wrote:

> > 
> 
> Well he did say “special stuff”. He only needs old new or super  
> there if he plans on initializing the object 😉

Yes. I’m writing an acts\_as\_mock plugin for rails controller tests.  
Since activerecord relies on a bunch of class methods, I need to be  
able to mock them as well so I can write tests like this:

def test\_blah  
expected = Story.new  
Story.should\_receive(:new).and\_return(expected)  
expected.should\_receive(:some\_message)

```
 get :blah

```

end

This (in theory) will allow us to run controller (functional) tests  
in isolation (i.e. no DB necessary), and therefore much faster.

In the example above, the first call to :new gets handled by super.  
The second line tells the class that the next time :new is called, it  
should return this particular object (expected). After that, you can  
set expectations on expected.

Thanks to your help here, this is working like a charm!

I’ve got a lot more work to do on this to make it even alpha-release  
worthy, but I’ll post a note here when I get there.

Cheers,  
David

---

<div class="post-metadata">

**Author:** ![crucoba](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/e8c25b/32.png) [@crucoba](https://www.ruby-forum.com/u/crucoba)\
**Post date:** [July 17, 2006, 11:34am UTC](https://www.ruby-forum.com/t/alias-new/98045/6 "2006-07-17T11:34:35Z")

</div>

David C. wrote:

> On Jul 12, 2006, at 12:15 AM, Logan C. wrote:  
> Yes. I’m writing an acts\_as\_mock plugin for rails controller tests.  
> Since activerecord relies on a bunch of class methods, I need to be  
> able to mock them as well so I can write tests like this:
> 
> def test\_blah  
> expected = Story.new  
> Story.should\_receive(:new).and\_return(expected)  
> expected.should\_receive(:some\_message)
> 
> ```
> get :blah
> 
> ```
> 
> end
> 
> This (in theory) will allow us to run controller (functional) tests  
> in isolation (i.e. no DB necessary), and therefore much faster.

Sounds interesting. I’ve been working on something similar - a mocking  
library called Mocha ([http://mocha.rubyforge.org](http://mocha.rubyforge.org)). It’s not specifically  
targetted at Rails controller tests, but I think the Stubba/AutoMocha  
parts might allow you to do what you want.

Stubba allows you to mock or stub class methods.

For AutoMocha, I alias the :new method in Mocha:MockClass  
([http://mocha.rubyforge.org/classes/Mocha/MockClass.html](http://mocha.rubyforge.org/classes/Mocha/MockClass.html)) and define a  
new version that delegates to method\_missing which is at the heart of  
the mocking functionality. The inherited method then ensures that any  
subclasses get the original :new method back.

James.