1👍
It depends on your scenario and the way you want to use.
Whenever you have to create an object / row inside the database then you will have to pass the whole object. assuming that the cars has already been created inside the database and you only have to link the booking object with the car than i will prefer you to bind it using ids rather than placing the whole object.
If you place the whole car object than it will increase the retrieval performance but if someday the information of car is changed than you will have to change the information on every booking object in which you placed the car object but if you just bind it with the Car Id than it will always give user the latest information even if the info of Car has been changed.
I thought that it will reduce the performance of the application if we used Id but that is not the case the DBMS use indexing to manage the search and search through id takes not time. It performance is not an issue and yes if you pass a whole object than it will also consume more data (since a large amount of data will be uploaded).