Another example for Mel...

gistfile1.rb

# Mel, Here is another gist for you, perhaps slightly better than the last.
# First, the migrations are exactly the same, but this improves the left-right
# situation slightly.

# kit.rb
class Kit < ActiveRecord::Base
  has_many :peer_kit_relations,
           :class_name => "KitRelation",
           :foreign_key => :left_kit_id
  
  has_many :peer_kits,
           :through => :peer_kit_relations,
           :source => :right_kit
  
  def associate_kit(other_kit)
    peer_kits << other_kit
    other_kit.peer_kits << self
  end
end

# kit_relation.rb
class KitRelation < ActiveRecord::Base
  belongs_to :left_kit, :class_name => "Kit"
  belongs_to :right_kit, :class_name => "Kit"
end


# exercising from rails console
k1 = Kit.create(:name => "First Kit")
k2 = Kit.create(:name => "Second Kit")
k3 = Kit.create(:name => "Third Kit")

# Note that this works:

k1.peer_kits << k2

# but the problem is the relationship goes one way only - so while k2
# is in k1's peer list, k1 is not in k2's peer list.  This means every
# time you have to do this:

k1.peer_kits << k2
k2.peer_kits << k1

# To help with that, there is the associate_kit method:

k1.associate_kit(k2)

# I still consider that an ugly solution.

# There should be a way to push knowledge of this bi-directional
# 2-record creation into the peer_kit_relations, using something
# like a block passed to the relationship:

  has_many :peer_kit_relations,
           :class_name => "KitRelation",
           :foreign_key => :left_kit_id do
    
    # some kind of magic goes in this block
    
  end

And one last disgusting thought -

Using the same tables, we could have the kit maintain a left_kits and a right kits just like the previous gist, but then create a custom view with a migration using straight sql something like:

create view peer_kits as
select left_kit_id, right_kit_id
from kit_relations
union
select right_kit_id, left_kit_id
from kit_relations

and then in the kit, define a method like this:

def peer_kits
find_by_sql "select * from kits where id in (select right_kit_id from peer_kits where left_kit_id = #{id})"
end

This would be more like the first solution, except the really heinous peer_kits method in ruby is pushed to a view and custom sql. I'm too tired to give that a shot at the moment...

Ok, I lied - I wasn't too tired to try it. I'm posting the new code to the old gist over at https://gist.github.com/1912278