Unable to import Model defined in another app - django

Question

I have two apps appointments and clinic in my project myappointments.
I have defined a model in appointments/models.py:
class Clinicdb(models.Model):
clinicid = models.AutoField(primary_key=True, unique=True)
name = models.CharField(max_length=60, unique=True)
label = models.SlugField(max_length=25, unique=True)
email = models.EmailField(max_length=50, default='')
mobile = models.CharField(max_length=15, default='')
alternate = models.CharField(max_length=15, default='', blank=True)
about = models.CharField(max_length=250, blank=True)
state = models.CharField(max_length=25)
city = models.CharField(max_length=35)
locality = models.CharField(max_length=35)
pincode = models.IntegerField(default=0)
address = models.TextField(max_length=80, default='', blank=True)
website = models.URLField(blank=True)
logo = models.ForeignKey(ProfilePic, blank=True, null=True, on_delete=models.CASCADE)
class Meta:
unique_together = ["name", "mobile", "email"]
def __str__(self):
return self.name
In clinic/models.py, I have:
from appointments.models import Clinicdb
class Album (models.Model):
name = models.CharField(max_length=255)
photo = models.FileField(upload_to="data/media/%Y/%m/%d")
clinicid = models.ForeignKey(Clinicdb, blank=True,
null=True, on_delete=models.CASCADE)
def __str__(self):
return self.photo
However when running makemigrations (and runserver) I get:
joel#hp:~/myappointments$ python3 manage.py makemigrations
Traceback (most recent call last):
File "manage.py", line 15, in <module>
execute_from_command_line(sys.argv)
File "/home/joel/.local/lib/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
utility.execute()
File "/home/joel/.local/lib/python3.6/site-packages/django/core/management/__init__.py", line 357, in execute
django.setup()
File "/home/joel/.local/lib/python3.6/site-packages/django/__init__.py", line 24, in setup
apps.populate(settings.INSTALLED_APPS)
File "/home/joel/.local/lib/python3.6/site-packages/django/apps/registry.py", line 112, in populate
app_config.import_models()
File "/home/joel/.local/lib/python3.6/site-packages/django/apps/config.py", line 198, in import_models
self.models_module = import_module(models_module_name)
File "/usr/lib/python3.6/importlib/__init__.py", line 126, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "<frozen importlib._bootstrap>", line 994, in _gcd_import
File "<frozen importlib._bootstrap>", line 971, in _find_and_load
File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 678, in exec_module
File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
File "/home/joel/myappointments/appointments/models.py", line 4, in <module>
from clinic.models import Pic, ProfilePic
File "/home/joel/myappointments/clinic/models.py", line 3, in <module>
from appointments.models import Clinicdb
ImportError: cannot import name 'Clinicdb'

Answer 1

You made a cyclic import, if we look at the bottom of the traceback, we see that:
File "/home/joel/myappointments/appointments/models.py", line 4, in <module>
from clinic.models import Pic, ProfilePic
File "/home/joel/myappointments/clinic/models.py", line 3, in <module>
from appointments.models import Clinicdb
So that means that clinic/models.py needs to import appointments/models.py first, but vice versa also holds, hence we can not import this. It comes down to the "chicken and the egg" problem: for an egg, we need a chicken, but for a chicken we need an agg, which results in a logical problem.
What you thus need to do is remove the import, and use a qualified name in a string, like:
# clinic/models.py
# NO import
class Album (models.Model):
name = models.CharField(max_length=255)
photo = models.FileField(upload_to="data/media/%Y/%m/%d")
clinicid = models.ForeignKey('appointment.Clinicdb', blank=True,
null=True, on_delete=models.CASCADE)
def __str__(self):
return self.photo
Django will automatically load the INSTALLED_APPS and replace the qualified string with a reference to the model (in fact one of the reasons of this functionality is solve the circular reference problem).
Note: normally ForeignKeys (and related relation fields) do not end with an id, or _id suffix, so I suggest that you rename clinicid to clinic. This makes sense since a some_album.clinic is not an id of a Clinic, but a Clinic object. Django will automatically add an extra field named clinic_id that stores the id.