Purposely update only the first in a collection of records

Purposely update only the first in a collection of records

I'm trying to build a simple inventory management of printer resources for my school. One table contains the Inventory of our printer cartridges; each individual cartridge has its own record - so, for example, there are three records, one for each of the three Q1234 cartridges in stock. I want to be able to update only one of those records - when a patron "checks out" one cartridge of that model, the records matching that cartridge number are fetched (multiple records), but only the first of those records needs to be updated with a check-out date. Two records will remain un-updated.

How can I select only one of a collection of records? I tried adding a numeric index, similar to arrays:

  1. rec = Inventory[Cartridge == input.Cartridge];
  2. rec[0].Date_Out == input.Date_Out;

with no success.