db.txt

USER
	username:string
	name:string
	administrator:boolean
	
	has_many :appointments
	
SLOT
	name:string
	bookable_id:integer
	start_time:integer
	end_time:integer
	
	has_many :appointments
	belongs_to :bookable

BOOKABLE

	has_many :slots
	has_many :appointments, :through => :slots

APPOINTMENT
	user_id:integer
	slot_id:integer
	kind_id:integer
	start:datetime
	end:datetime
	
	belongs_to :user
	belongs_to :slot
	has_one :bookable, :through => :slot
	has_one :owner, :through => :bookable
	belongs_to :kind

KIND
	name:string
	
	has_many :appointments