## lib/ext/exclusive_scope.rb

module ActiveRecord
  module NamedScope
    class ExclusiveScope < Scope
      def initialize parent_scope, options = {}
        super(parent_scope, options)
      end

      private

      def method_missing *args, &block
        with_scope({}, :overwrite) { super }
      end
    end
  end
end

class << ActiveRecord::Base
  def exclusive_scope options = {}
    ActiveRecord::NamedScope::ExclusiveScope.new self, options
  end
end

## app/models/event.rb

class Event < ActiveRecord::Base
  default_scope :conditions => { :campaign_id => nil }
  # ...

  def self.campaign campaign
    ActiveRecord::NamedScope::ExclusiveScope.new(self, :conditions => { :campaign_id => campaign })
  end

  # ...
end

## console

>> Event.all
=> []
>> Event.campaign Campaign.first
=> [#<Event id: 1, name: "foo", created_at: "2009-11-25 22:34:26", updated_at: "2009-11-25 22:34:26", user_id: 1, start_time: "2009-11-25 22:34:26", end_time: "2009-11-26 00:34:26", description: nil, metro_area_id: nil, location: nil, street_address: nil, address_line_2: nil, city: nil, us_state: nil, zip_code: nil, delta: true, reminded_at: nil, campaign_id: 1>]
>>