One of the reasons I love MetaWhere (I may be biased :))

verb.rb

# Can this be done with strings? Sure. This feels much nicer to me, though.
class Verb < ActiveRecord::Base
  belongs_to :verbable, :polymorphic => true
  belongs_to :objectifiable, :polymorphic => true

  scope :of_interest_to_user, lambda {|u|
    where(
      (:objectifiable_type.eq % 'User' & :objectifiable_id.eq % u.id) |
      (:objectifiable_type.eq % 'Book' & :objectifiable_id.in % u.book_ids)
    )
  }
end


# => SELECT `verbs`.* FROM `verbs` WHERE (
#     (`verbs`.`objectifiable_type` = 'User' AND `verbs`.`objectifiable_id` = 1 
#      OR `verbs`.`objectifiable_type` = 'Book' AND `verbs`.`objectifiable_id` IN (1, 2, 3, 4))
#    )