here are the model associations

```
class Driver
  has_many :cars

  validates :cars, presence: true
end

```

```
class Car
  belongs_to :driver
end

```

The car records already exists. It does not need to have a driver

first car with id 1, gets assigned to driver 1

```
driver1.car_ids = [1]

```

driver1 now has a car

Now we create driver 2 with car 1

```
driver2.car_ids = [1]

```

driver1 becomes invalid, because it no longer has cars

What is the best way to handle this?