Problem with primary key field that needs updating on commit

Question

Taking on someone else's form and trying to tidy it up as it has no primary key on tha table.
The only primary key i can apply is (invoice number, line number)
This has to be displayed on the form as a provisional number when the user enters the form. So it need to show for example inv number 1000 with each line on the invoice being 1,2,3,4 etc.
The form then has a linked table for the lines per invoice that inserts invnumber and line number into a seperate table on PRE INSERT trigger. This means that if they put on various lines i need to do a post.
My problem comes because more than one person can use the form at the same time so user a will go into the form and see inv number 1000, user b will also see 1000. The original form was making these null and updating to correct number on commit but i wanted to add a primary key.
Im jusat wodering the best way to do this as if user a does not save before user b. there is a potential for a lock to occur on the post. Also they must be updated to a new number if someone else saves before them

Answer 1

either you create a sequence for this by create sequence command and populate the sequence value. but in this case if the form is cancelled the sequence is missed and the next sequence will appear. Another way to do is to generate the invoice number when the user press save button and then assign that value to text item. It is the best way to do. So the user will see invoice number only when they save and it happens in milli seconds so there is no question of overlapping invoice number.
Hope this helps.