unable to create One to one relationship using the look up field

Hi all

I am new to zoho and database so i need your guidance

I have one two forms "JOBS" form where each job has unique job number

and "Invoice" form where each invoice has a unique invoice number

Each job can only relate to only one invoice and vice versa

I have linked by creating look up field in JOBS form so that we can look up for Invoice Number.

The problem i am having is that one invoice number can be selected in many jobs

Please help how can i relate two forms so that one job is only linked to one invoice