Disable/hide drop-down options if that option exists in another record


I am setting up a sign-in/out form, with which a user will input their details and select a key from a drop-down box. Those keys are recorded in an object which is connected to the sign-in/out form object.

After signing in, users will edit their record to insert a sign out date/time. What I am hoping to achieve is to make keys which are associated with a user that has not yet signed out unavailable to other users.

E.g. Steven signs in on 12 September and selects key 1. The sign-out date/time field is left blank. Peter then signs in and is only able to select keys 2-10 - he selects key 10. Steven then signs out and returns key 1. John then signs in and is able to select keys 1-9.

If possible, I'd like to do this without any coding - those who will be maintaining our apps into the future will likely not have an understanding of languages like Javascript.



If I understand correctly, why don't you add a field (Available) to the drop down record, and when its checked out set it to No, when it's checked in set it to Yes, and then filter on Yes in the drop down?