Standart Operations
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