Rails - Nov 2006 - alias_method_chain :<<

Hi

I''d like to modify the behavior of the  << method so that
duplicates are not
appended to my AR objects association.

I thought I could just use alias_method_chain on it but I can''t seem to
make
the right method for it let alone make the call to alias_method_chain.

I tried in the console.
>> a = ""
=> ""
>> def a.<<_with_extra( *args )
puts "Test"
end

so that I could use
alias_method_chain :<<, :extra

How is this done?

Cheers
Daniel


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Hi --

On Sun, 26 Nov 2006, Daniel N wrote:
> Hi
>
> I''d like to modify the behavior of the  << method so that
duplicates are not
> appended to my AR objects association.
>
> I thought I could just use alias_method_chain on it but I can''t
seem to make
> the right method for it let alone make the call to alias_method_chain.
>
> I tried in the console.
>
>>> a = ""
> => ""
>>> def a.<<_with_extra( *args )
> puts "Test"
> end
>
> so that I could use
> alias_method_chain :<<, :extra
>
> How is this done?
I think you''re going to have to use a word, like
"append_extra".
While << is a legitimate method name, <<_other_stuff is parsed as a
here-document.

Of course, you can smuggle in weird method names with define_method
and send:

   class << a
     define_method("<<_with_extra_args") {|*args| ... }
   end

   a.send("<<_with_extra_args",x,y,z)

but that of course is awkward and opaque.


David

-- 
                   David A. Black |
dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org
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

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You can do it, but you''ve got to be careful because you cant simply
write

def <<_without_extra(*args)

Ruby sees the "<<" as the beginning of a heredoc. Something like
this should
work, making sure method names starting with "<<" are defined
with strings:

class Array
  define_method "<<_with_extra" do |*args|
    # Do whatever you like here
    send("<<_without_extra")
  end

  alias_method_chain :<<, :extra
end

That should work as expected, but there may be a better way to stop
duplicates being added to the association. If it is a has_many :through you
could add uniqueness validation on the join model, or if it''s
has_and_belongs_to_many you could use the :uniq option on the association.

-Jonathan.

On 11/26/06, Daniel N <has.sox-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> Hi
>
> I''d like to modify the behavior of the  << method so that
duplicates are
> not appended to my AR objects association.
>
> I thought I could just use alias_method_chain on it but I can''t
seem to
> make the right method for it let alone make the call to alias_method_chain.
>
> I tried in the console.
>
> >> a = ""
> => ""
> >> def a.<<_with_extra( *args )
> puts "Test"
> end
>
> so that I could use
> alias_method_chain :<<, :extra
>
> How is this done?
>
> Cheers
> Daniel
>
> >
>

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On 11/26/06, Jonathan Viney
<jonathan.viney-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> You can do it, but you''ve got to be careful because you cant
simply write
>
> def <<_without_extra(*args)
>
> Ruby sees the "<<" as the beginning of a heredoc. Something
like this
> should work, making sure method names starting with "<<"
are defined with
> strings:
>
> class Array
>   define_method "<<_with_extra" do |*args|
>     # Do whatever you like here
>     send("<<_without_extra")
>   end
>
>   alias_method_chain :<<, :extra
> end
>
> That should work as expected, but there may be a better way to stop
> duplicates being added to the association. If it is a has_many :through you
> could add uniqueness validation on the join model, or if it''s
> has_and_belongs_to_many you could use the :uniq option on the association.
>
> -Jonathan.

It is a has_many :through, but  I didn''t realise that the :uniq option
would
prevent duplicates from being added.  That''s all I''m really
after.  I guess
I should realize by now that these kinds of things are already easy!

Cheers


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