try changing the sifra field in the software table to "unique" in your field definitions and look at your graph again.
Thank you for answer! Do you know can I somehow mark different primary and foreign key!?
prefix the primary key name with pk and the foreign key with fk
Since the key fields all seem to begin with ID in your current system, an alternative is to use ID and IDf, like this:
ID_invoice (primary key)
IDf_invoice (foreign key)
ID_contact (primary key)
IDf_contact (foreign key)