'Django: Non-primary Foreign Key object can't access related model instance

I'm new in Django. I have 2 class tech_system adn equiptment in models.py

class tech_system(models.Model):
    id_tech_system = models.BigAutoField(db_column='ID_tech_system', primary_key=True)
    system_descript_short = models.CharField(max_length=255, blank=True, null=True)
    #More field here
    tech_system_code = models.CharField(unique=True, max_length=40)

    class Meta:
        managed = False
        db_table = 'tech_system'
        
    def __str__(self):
        return self.system_descript_short

        
class equiptment(models.Model):
    id_thietbi = models.BigAutoField(db_column='ID_thietbi', primary_key=True)
    tech_system_code = models.ForeignKey('tech_system', models.DO_NOTHING, db_column="tech_system_code", blank=True, null=True)

    class Meta:
        managed = False
        db_table = 'equiptment'

I use python shell, equiptment model object can't access to related tech_system model instance. I got the error matching query does not exist. I want to get the value obj1.equiptment.tech_system_code.system_descript_short. How can I do?

Thank you.

>>> obj1 = equiptment.objects.first()
>>> obj1.tech_system_code_id
'530'
>>> obj1.tech_system_code
Traceback (most recent call last):
  File "D:\Dev1\env1\lib\site-packages\django\db\models\fields\related_descriptors.py", line 173, in __get__
    rel_obj = self.field.get_cached_value(instance)
  File "D:\Dev1\env1\lib\site-packages\django\db\models\fields\mixins.py", line 15, in get_cached_value
    return instance._state.fields_cache[cache_name]
KeyError: 'tech_system_code'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "D:\Dev1\env1\lib\site-packages\django\db\models\fields\related_descriptors.py", line 187, in __get__
    rel_obj = self.get_object(instance)
  File "D:\Dev1\env1\lib\site-packages\django\db\models\fields\related_descriptors.py", line 154, in get_object
    return qs.get(self.field.get_reverse_related_filter(instance))
  File "D:\Dev1\env1\lib\site-packages\django\db\models\query.py", line 437, in get
    self.model._meta.object_name
app.models.tech_system.DoesNotExist: tech_system matching query does not exist.

Solution 1:^[1]

I found that tech_system_code record is assigned as str type, not an object in equiptment model. Record values imported to the database is not correct. Thanks for your help.

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Solution	Source
Solution 1	QuangNA

'Django: Non-primary Foreign Key object can't access related model instance

Solution 1:[1]

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Solution 1:^[1]