From: dohzya <dohzya@...>
Date: 2007-07-25T03:38:17+09:00
Subject: Bidirectionnal relation

Hi everyone,
I have a problem with bidirectional relations : how to implements it ?

I can't implements it with 2 relations without synchronize them :
---
# no synchronization
class Entity
  attr_accessor :mother
  attr_reader :daughters
  def initialize
    @daughters = []
  end
end

a = Entity.new
b = Entity.new
a.mother = b
puts (b.daughters.include? a) # false
a.daughters << b
puts (b.mother == a) # false
---
too bad...

Try with a simple synchronization :
---
# simple synchronization
class Entity
  attr_reader :mother, :daughters
  def initialize
    @daughters = []
  end
  def mother= m
    @mother.daughters.delete self unless @mother.nil?
    @mother = m
    @mother.daughters << self unless @mother.nil?
  end
  def add_daughter d
    unless d.nil?
      @daughters << d
      d.mother = self
    end
  end
end

a = Entity.new
b = Entity.new
a.mother = b
puts (b.daughters.include? a) # true
a.daughters << b # it's possible so why not use it ?
puts (b.mother == a) # false
---
ok for some operations, but not exhaustive.

The problem seems to come from @daughters which is a simple Array.
Let's fix it :
---
# better synchronization
class Entity
  attr_reader :mother, :daughters
  def initialize
    @daughters = []
    class << @daughters
      attr_accessor :mother
      alias __old_add__ <<
      def << d, recc=false
        __old_add__ d
        d.mother = @mother unless recc
      end
      alias __old_delete__ delete
      def delete d
        if include? d
          d.mother = nil 
          __old_delete__ d
        end
      end
    end
    @daughters.mother = self
  end
  def mother= m
    @mother.daughters.delete self unless @mother.nil?
    @mother = m
    @mother.daughters.<<(self, true) unless @mother.nil?
  end
  def add_daughter d
    unless d.nil?
      @daughters << d
      d.mother = self
    end
  end
end

a = Entity.new
b = Entity.new
a.mother = b
puts (b.daughters.include? a) # true
a.daughters << b
puts (b.mother == a) # true
---
ok it works (I think), but I need to redefine all methods from
@daughters...

26 lines more than the first (and natural) solution, for a really simple
problem... I don't think I'll often use it.

I tried to implement a solution with a real relation, but I had too many
problems.

If anyone has a suggestion (solution or link or anything) to implement
transparent bidirectional relation... :)
Thanks !

--
Etienne Vallette d'Osia