Counting number of records from related module

Counting number of records from related module

Hi,

I have a field called: 'Number of unsolved issues'. I want this field to contain the number of records (from module 'Debiteuren') which are not solved. How can I do this? How can I update this field? So in this case, the field should display the number 12. However, it should be updated weekly, because the number of unsolved issues could decrease to for example 6