dm-oddity.rb

class Message
  include DataMapper::Resource

  property :id,      Serial
  property :deleted, Boolean

  belongs_to :sender,
    'User'

  belongs_to :recipient,
    'User'

  class << self
    def visible
      all(:deleted => false)
    end
  end
end

class User
  include DataMapper::Resource

  property :id, Serial

  def messages
    Message.all(:sender => self) | Message.all(:recipient => self)
  end
end

# this works (returns records from the current query that have :deleted => false)
user = User.get(1)
user.messages.all(:deleted => false).count

# this doesn't (returns *all* records with :deleted => false... not scoped to the current query)
user.messages.visible.count