Python Django always passes id 1 on ForeignKey with to_field

I'm new to Django and trying to create a small application that shows scanned data from virtual machines that are inserted in a table named HostsFixDataScans.

To access the scanned data in HostsFixDataScans via the Hosts model, I defined a ForeignKey with to_field.

But unfortunately, the data returned by the linked HostsFixDataScans are wrong.

I checked the SQL statements and when requesting the HostsFixDataScans table, not the id of the Host is used, but always 1.

My domain = models.ForeignKey(Domains, on_delete=models.CASCADE) definition which does not use to_field works correctly.

I'm pretty sure, I have a misunderstanding of the definition of this relationship.

Maybe you could give me some hints how to solve the problem? Many thanks in advance!

Here are the shortened definitions of the models:

class Os(models.Model):
    operatingsystem = models.CharField(max_length=32)
    lsbdistcodename = models.CharField(max_length=32)
    lsbdistrelease = models.CharField(max_length=32, db_collation='ascii_bin')
    lsbmajdistrelease = models.IntegerField()
    remarks = models.CharField(max_length=256, blank=True, null=True)

class HostsFixDataScans(models.Model):
    host_id = models.PositiveIntegerField(primary_key=True, unique=True)
    scan_id = models.PositiveIntegerField()
    os = models.ForeignKey(Os, on_delete=models.CASCADE)

class Hosts(models.Model):
    hostname = models.CharField(max_length=256)
    domain = models.ForeignKey(Domains, on_delete=models.CASCADE)
    is_virtual = models.PositiveIntegerField(blank=True, null=True)
    os = models.ForeignKey(HostsFixDataScans, to_field='host_id', on_delete=models.CASCADE)