Ruby on Rails - has_one relation, how to check if it has an existing association?

Question

I have a simple problem, related to associations. I have a model for book, that has_one reservation. Reservation belongs_to book.

I want to make sure in the create method of reservations controller that a book is not reserved already when a reservation is made. In other words, I need to check if any other reservation exists for that book. How do i do that?

EDIT: Aaaand i made it, thanks everyone for the tips, learned some new stuff. When i tried offered solutions, I got no_method errors, or nil_class etc. That got me thinking, that the objects I'm trying to work on simply don't exist. Krule gave me the idea to use book.find, so i tried working with that. Ultimately i got it working with:

book=Book.find_by_id(reservation_params[:book_id])
unless book.is_reserved?

Thanks everybody for your anwsers, I know it's basic stuff but i learned a lot. Cheers!

Smart

Hey, i tried book.reservation.any? and similar constructs. I think the problem is, object Book is not present in the reservations controller. The book id is passed as a parameter from a different controller. – Smart

Better edit the question to add that data. – Crayfish

Answer 1

#app/models/book.rb

def is_reserved?
  !self.reservation.nil?
end

# Somewhere else
book = Book.find(id)
book.is_reserved?

Etiolate

Now getting error: undefined local variable or method `id' for #<ReservationsController:0x000000050ef3f8> I think the books are not visible in the reservations controller. The book id is passed from different controller as a parameter. – Smart

You should use an actual book id variable instead of just writing id without instantiating it as variable. Something along the lines of id = params[:id] in controller or numeric value representing it should do the trick. – Etiolate

Answer 2

How about:

self.reservation.present?

This should return true if an association exists.

Indian

This has the unnecessary side effect of fetching the entire Reservation record from the database and instantiating it as an ActiveRecord instance, which is not necessary in order to find out if it exists. – Cur

Answer 3

Little bit performance gain can be obtained if you use

#app/models/book.rb
def is_reserved?
  Reservation.exists?(book_id: id)
end

#somewhere else
book = Book.find(id)
book.is_reserved?

Torn

Answer 4

Simply, use:

# book = Book.first
book.reservation.nil?   # returns true if no reservation
                        # is associated with this book

Hereunto

It give me error: undefined local variable or method `book' for #<ReservationsController:0x0000000546a258> – Smart