Question

When testing a model with a foreign key, I’d like to assert the model can’t be saved with an inexistent foreign key.

Example class for testing:

class Wheel
    belongs_to: car
end

So, a unit test would look like this:

def test "a wheel must belong to an existent car"
    @wheel = Wheel.new
    @wheel.car_id = INEXISTENT_CAR_ID
    assert !@wheel.save
end

What is the best way to find a valid INEXISTENT_CAR_ID (knowing fixtures are loaded with random ids)?

Answer 1

I like Chowlett’s approach. But it fails, if there is no record at all, so you might want to write

@wheel.car_id = Car.order("id").last.try(:id).to_i + 1