问如何在Django中将unique_together约束从一组模型字段更改为另一组

EN

class TenderOrigin(models.Model):
    doc_type = models.CharField(max_length=2, ...)
    doc_short_txt = models.CharField(max_length=150, ...)
    create_date = models.DateField(default=timezone.now, ...)
    port_grp = models.ForeignKey(PortGroup, on_delete=models.CASCADE, null=True, ...)
#    frt_grp = models.ForeignKey(FrtGroup, on_delete=models.CASCADE, null=True, ...)
    locn_from = models.ForeignKey(Plant, on_delete=models.CASCADE, ...)
    tender_number = models.CharField(max_length=150, null=True,...)
    tender_date = models.DateField(null=True, ...)

    class Meta:
        ordering = ['locn_from__LocName']
#        unique_together = [['frt_grp', 'locn_from', 'tender_date'],]    # Original constraint
        unique_together = [['port_grp', 'locn_from', 'tender_date'],]    # New constraint being defined

...
...    
Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
...
...
 File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/django/db/backends/sqlite3/base.py", line 303, in execute
    return Database.Cursor.execute(self, query, params)
django.db.utils.IntegrityError: UNIQUE constraint failed: matmovt_tenderorigin.port_grp_id, matmovt_tenderorigin.locn_from_id, matmovt_tenderorigin.tender_date