n<--extra_column-->n relationships...

You are describing a "has_many :through" relationship.

class FlavoursPacks < ActiveRecord::Base
  belongs_to :flavour
  belongs_to :pack
end

class Flavour < ActiveRecord::Base
  has_many :flavours_packs
  has_many :packs, :through => :flavours_packs
end

class Pack < ActiveRecord::Base
  has_many :flavours_packs
  has_many :flavours, :through => :flavours_packs
end

Then you can access flavours_packs from either direction and call
methods on those objects.

somePack.flavours_packs[0].article_number

-- James