Alias_method_chain :<<

Rails
1

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

2

Hi –

On Sun, 26 Nov 2006, Daniel N wrote:

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 | [email protected]
Author of “Ruby for Rails” [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB’s Weblog) [2] | Co-director, Ruby Central, Inc. [4]
[1] Ruby for Rails | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com | [4] http://www.rubycentral.org

3

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.

4

On 11/26/06, Jonathan V. [email protected] wrote:

define_method “<<_with_extra” do |*args|
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