I'm creating a book catalogue with loaning history for each book.
The book catalogue shows a browseedit form named "frm_knihy", background table is "tbl_knihy" which includes a field named "tbl_pozicky_id" planned as foreign key for loanings.
The loaning history shows a form named "frmsub_pozicky", background table is "tbl_pozicky" which includes a field named "poz_id" as a primary key .
I'm trying to include a subform object "frmsub_pozicky" indo edit page of "frm_knihy", but when it comes during setting the subform to the foreign key, dropbox with available fields doesn't contain the foreign key "tbl_pozicky_id" from the main form frm_knihy. I'm not able to finish the subform setting

What I'm doing wrong ?
Thanx in advance.