Rewrite has_many association on the fly

group.rb

# Ever needed to dynamically rewrite the conditions on a has_many association?
# This is definitely a hack, but useful if parameterizing or adding scopes are
# not practical for whatever reason.

class Group < ActiveRecord::Base
  # Any association will do
  has_many :people

  def filter_people(conditions)
    # All calls to this method after the association is traversed and before a reload
    # are idempotent.  You can reload manually, if you prefer.
    self.reload

    # You need to specify the foreign key because (I think) this breaks reflection.
    (class << self; self; end).instance_eval do
      has_many :people, :conditions => conditions, :foreign_key => :group_id
    end
  end
end