Change Values in Controller A for Domain Class B

[numxxx]

Hi,

im newbie in grails. I use three Domain Class Books, Customers and Rents.

My Question:

It is possible if i create a new rent (the rent create use private key from Domain Class Customers and Books) and before i save the new rent that i change a field value in Domain Class Books?

Example: I create a new rent. A rent use the Data from the Customer and the Book that customer want rent. If the rent is create the value for book count should reduce for this rent book (Example from 15 to 14 books).

If this possible how should i do this. I test with this in Controller Rent

def changeBooks(Long id) {
     def p=Books.get(id)
      p.properties
      Books.executeUpdate("update Books p set p.anzahl=anzahl -1")
     redirect view:'edit'

but i get the message


URI
    /rents/changeBooks
Class
    java.lang.NullPointerException
Message
    null
Caused by
    Cannot get property 'properties' on null object


I use the mem H2 DB

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