Regarding lock objects

I've a TCode to modify a report. That TCode picks a doc no. from a table & opens up an editable ALV based on that doc. no .That doc. no. is the only primary key.

I want that if someone is already opened that report, & if someoen else is trying to open that report, it shouldn't open & should show that its locked. I've made a lock object for that table. Please advise what to do next.