Question

In my Rails app I have classes that depend on one another, ParkingSpace and Parking Lot.

A ParkingLot has many ParkingSpaces and the ParkingSpace belongs to a ParkingLot.

I want to ensure that the same ParkingSpace can’t be inserted into the ParkingLot again.

Typically, I would use a uniqueness validation to ensure that the ParkingSpace can’t be repeated parking space have very generic names

ParkingLot with id:1 can have ParkingSpaces 1, 2 and 3 on it.

ParkingLot with id:2 can also have ParkingSpaces 1, 2 and 3 on it.

But the above parking lots should not be able to have two parking spaces with the same name.

How can I validate this in Rails?
How can I validate this at the database level?

I’m considering joining the two fields together in a unique way and then calling unique on that. Is this a valid approach? (Not exactly sure how to do this at the database level, but all my validations need to be present there.)

(using Postgres)

Answer 1

Assuming your parking space and an identifier (like a name), you would do something in the ParkingSpace class like:

class ParkingSpace < ActiveRecord::Base

  belongs_to :parking_lot

  validates_uniqueness_of :name, :scope => :parking_lot_id

end