class Campaign::Event < Event
  belongs_to :campaign
  default_scope {}

  named_scope :not_nil, lambda { |col| { :conditions => [ "#{col} IS NOT NULL" ] } }
end

>> Campaign::Event.not_nil("id").all(:joins => :campaign)
ActiveRecord::StatementInvalid: Mysql::Error: Column 'id' in where clause is ambiguous: SELECT `events`.* FROM `events`   INNER JOIN `campaigns` ON `campaigns`.id = `events`.campaign_id  WHERE (id IS NOT NULL)