blocking.rb

class Blocking
  include DataMapper::Resource

  property :task_id, Integer, :required => true, :index => true, :key => true
  property :blocker_id, Integer, :required => true, :index => true, :key => true
  
  belongs_to :task
  belongs_to :blocker, :model => Task
end

gistfile2.txt

Task.first.blockers

DataObjects::SQLError: Not unique table/alias: 'tasks' (code: 1066, sql state: 42000, query: SELECT `tasks`.`id`, `tasks`.`title` FROM `tasks` INNER JOIN `blockings` ON `tasks`.`id` = `blockings`.`blocker_id` INNER JOIN `tasks` ON `blockings`.`task_id` = `tasks`.`id` WHERE `blockings`.`task_id` = 1 GROUP BY `tasks`.`id`, `tasks`.`title` ORDER BY `tasks`.`id`

task.rb

class Task
  include DataMapper::Resource

  property :id, Serial
  property :title, :required => true, :length => 256

  has n, :blockings
  has n, :blockers, :through => :blockings, :model => Task
end