Missing Method as Filter

Let's say I have a model called User. User has two attributes or table
columns: id, username

By using the code below I can check that there is a username "Tom" and
return 1 ifthere is and 0 if there is not

User.find_by_username("Tom")? 1 : 0

How would I find out Tom's corresponding id?

For example I'm looking for something along the lines of:

That will work except that you should probably check that Tom exists,
as it will throw an error if he does not (as you would be effectively
saying nil.id).


(user = User.find_by_username("Tom")) ? user.id : 0