Your description sounds like you can't lock the door when you leave your apartment. After all, you can't access the inside doorknob's button when you are standing on the outside side of the door! Well, I must be misunderstanding something because that would be too funny. I have my own door's lock set so that I can't even close, let alone lock, the thing without a key from the outside. So, there's absolutely no risk of forgetting the key when leaving. I also have a second, high security lock that only obeys a big, sturdy key from both sides, to keep thieves away from my precious electronics.