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.