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 1. Directory.joins(:admin => :user)
 2. Directory.joins(:admin).joins("LEFT JOIN users ON users.id = pivots.user_id")
 # 1 and 2 => same thing.
 # directory belongs_to pivot(admin) and pivot belongs to user

rake db:migrate VERSION=20080906120000

rake db:rollback

rake db:migrate:redo STEP=3

rake db:migrate:up VERSION=20080906120000

rake db:migrate:down VERSION=20080906120000