Alias_method_chain vs. redefined method with super


#1

I’m trying to fully understand the subtleties of using
alias_method_chain to refine method behavior versus redefining a method
and calling super. I have a simple data model (a music collection) that
I often use to evaluate web frameworks, and I’m trying to get it working
on Rails 1.2.0RC1.

Specifically, I have three classes which subclass a common Artist class:

  • Band (name:string)
  • Musician (first_name:string, last_name:string)
  • Duo (first_artist:Artist, other_artist:Artist)

I’m using single table inheritance to store all of the fields for the
various Artist types in one table. I want the ‘name’ attribute to be
derived for Musicians (first_name+last_name) and Duos
(first_artist.name+last_artist.name) and persisted so I can sort all
Artists by name.

I added code to Musician which redefined first_name= as:

class Musician < Artist
def first_name= (first_name)
super(first_name)
update_name # sets name to first + last
end

end

For the Duo class, however, this didn’t work. It’s definition looks
like:

class Duo < Artist
belongs_to :first_artist, :class_name => ‘Artist’
belongs_to :other_artist, :class_name => ‘Artist’

end

When I tried to define first_artist= and call super as I did for
Musician, it complained that the method didn’t exist in super. I dug
around a bit and discovered alias_method_chain, so I finally ended up
with:

class Duo < Artist
belongs_to :first_artist, :class_name => ‘Artist’
belongs_to :other_artist, :class_name => ‘Artist’
def first_artist_with_update= (first_artist)
self.first_artist_without_update = first_artist
update_name # sets name to first_artist.name + other_artist.name
end
alias_method_chain :first_artist=, :update

end

Thinking that was a pretty cool way to extend a method, I went back and
tried to use the same scheme for Musician, but there I get the missing
method error again! Strange, since there’s definitely a first_name=
method, although it’s defined on Artist not Musician.

So, I guess I have two questions:

  1. Why couldn’t I call super within Duo.first_artist=, when :belongs_to
    is supposed to have added a method by that name? Is it because the
    additional methods are defined directly on Duo (and thus not really
    super-able)?

  2. Why can’t I use alias_method_chain in my Musician class? Does
    alias_method_chain only work for methods defined directly on the current
    class?

Having to do this two different ways depending on where the method in
question is defined seems a bit messy. I’m hoping there’s a single
scheme I’ve missed that will work in both cases.

Thanks, and sorry for the long post…

Dustin