update_attributes with validations

Question

I have the following contrived example in Rails. I want to make sure the Garage model has at least one car with this.

class Garage
    has_many :cars
    validate :at_least_one_car
    accepts_nested_attributes_for :cars, :allow_destroy => true

    def at_least_one_car
        if cars.count == 0
          errors.add_to_base("needs at least one car")
        end
    end
end

class Car
    belongs_to :garage
end

In my form I have a remove button that will set the hidden field _delete to true for an existing car. Let's say there is only one car object and I "delete" it in my form, if I do garage_object.update_attributes(params[:garage]), it will delete the car model and make the garage object invalid. Is there to a way to make it not update the attributes if it will make the model invalid?

params = {
  :garage => {
    :car_attributes => {
      ["0"] => {
        _delete => true,
        # other attributes here
      }
    }
  }
}

Answer 1

From the little i know your question following will help you.

You want some validation which should check when you creating object and not when you update it. use validate_on_create and validate_on_update methods for that

class Garage
    has_many :cars


    def validate_on_create
        if cars.count == 0
          errors.add_to_base("needs at least one car")
        end
    end
end

class Car
    belongs_to :garage
end

Hope that helps :)

EDITED

What you are using to delete the object

destroy or delete

when you are using 'delete' no callbacks are get fire off

when you are using 'destroy' all callbacks and filters are fired off before the object is deleted.

so check and use 'destroy'

Answer 2

I believe you can add a validates_presence_of :car, so the garage_object won't be updated if there's no associated cars.

class Garage
    has_many :cars
    validates_presence_of :cars
end

class Car
    belongs_to :garage
end