Django model choice in separate constant file - django

Question

According to django docs the best way of handling choice fields in models is do like this:
class Book(models.Model):
AVAILABLE = 'available'
BORROWED = 'borrowed'
ARCHIVED = 'archived'
STATUS = [
(AVAILABLE, 'Available to borrow'),
(BORROWED, 'Borrowed by someone'),
(ARCHIVED, 'Archived - not available anymore'),
]
# […]
status = models.CharField(
max_length=32,
choices=STATUS,
default=AVAILABLE,
)
However, my app grew a lot and model inter-connectivity between different app extended. In some cases I need to import the model class just to use the constant values
eg:
# some other file
from book.models import Book
def do_stuff_if_borrowed(book):
if (book.status == Book.BORROWED):
# do stuff
pass
I'd like to avoid importing models in other file as much as possible because it causes a lot of circular import issues and overall I feel it is not the cleanest way. I checked quickly and didn't find anybody presenting a solution on how to externalize these model constants like:
book/constants.py
AVAILABLE = 'available'
BORROWED = 'borrowed'
ARCHIVED = 'archived'
STATUS = [
(AVAILABLE, 'Available to borrow'),
(BORROWED, 'Borrowed by someone'),
(ARCHIVED, 'Archived - not available anymore'),
]
book/models.py
from .constants import STATUS, AVAILABLE
class Book(models.Model):
status = models.CharField(
max_length=32,
choices=STATUS,
default=AVAILABLE,
)
and then if I need only constants I don't need to imports Book model but only constants.py
Is there a reason not to do like this?

Answer 1

I am working on a project where we are considering something like this. It is connected to another service API which has metrics and definitions that we pull. So we had considered just pulling these files routinely and storing them in something like a metrics.json file and than using them in the models.
This is pretty fluid and nice, but I think what you lose is visibility to changes in models that result in migrations needing to be created, tracked and run.
In your instance, it seems like the constants won't really be changing and the intention is just to be DRY. It seems like you could create that constants file and then add a method the returns prefixed statuses for you model if you did want to have them be more distinct for each model.
What you sacrifice in this instance is easily referencing those values directly from the model. In the example from the docs you could reference the status using Book.STATUS_BORROWED. But in your example, you would have to actually import those constants separately.
I think the best solution is just to make a mixin that covers this and have all of your models that need these statuses inherit from that.
class BorrowableBookMixin(models.Model):
STATUS_AVAILABLE = 'available'
STATUS_BORROWED = 'borrowed'
STATUS_ARCHIVED = 'archived'
STATUS_CHOICES = [
(STATUS_AVAILABLE, 'Available to borrow'),
(STATUS_BORROWED, 'Borrowed by someone'),
(STATUS_ARCHIVED, 'Archived - not available anymore'),
]
status = models.CharField(
max_length=32,
choices=STATUS_CHOICES,
default=STATUS_AVAILABLE,
)
class Meta:
abstract = True