Quick, where would you put a high-level business function that touches multiple models? Where would you put the model-level validation for that? I've seen all of the following: in a view, in a form, in a service-layer function, arbitrarily in one of the models/managers, implicitly in the database schema (just catch the IntegrityErrror!).
There are endless discussions online about where to do this. This isn't a good sign. It should be easy. The business logic should be at the heart of an application. But instead we see Django's ORM at the heart.
Django models are really limiting and totally tied to the database. You can't even have a model that has a list/set of items without getting into ForeignKeys etc. You can't test any of it without having the database present. Why would I need the database to be there to test business logic?! The point of an ORM is to do object persistence, not to do business logic.
So I've seen people do a separate set of ORM models and domain models and then manually mapping between them. This is possible, but it's a whole lot easier with something like SQLAlchemy which is actually designed for that (it's a data mapper rather than active record type ORM).
Then there's stuff like django-admin and ModelForms etc. which are the last thing you want if you're doing more than CRUD.
Tbh it would be utterly remarkable if Django somehow made this stuff easy. But it doesn't and I find myself wishing I just put in the effort to set up the boring stuff like auth etc. but with a proper architecture.
You could use Django like this, but you'd end up manually mapping between Django models and high-level domain models. But this mapping is exactly what an ORM like SQLAlchemy does! It seems crazy to do it yourself. So nobody does. They build fat models with all the business logic right next to application logic like foreign keys etc.
Even if you did try to do something different developers would complain that it's no longer a "Django project". Have you ever seen any Django project do business logic completely independently of application logic?
If you really prefer SQLAlchemy, there is nothing stopping you from using it over the Django ORM, I happen to prefer the Django ORM, but Django doesn't get jealous if you go out with a different ORM (it is a polyamorious web framework[1]).
Yes, I have seen Django applications do that. There are plenty of individuals out there advocating such a separation. I wouldn't worry about these "different developers' and their opinions.
[1] I was going to use a smiley face here, but then I recalled that a Python dev was de-flocked recently for using smiley emoji as it is offensive to some individuals, so insert whatever words or symbols you don't find offensive that indicate humor instead.
It's just python: `descriptive_name.py`. `ingest_validation.py` etc. If this seems too terse, it's because your question was posed in a general way. If you have more info on the sort of business logic you have in mind, or the sort of data validation, I'll reply with more.
Could you describe what you have in mind regarding models not tied to the database? If it's the naive interpretation I'm thinking of, use python dataclasses and enums. Django models are specifically to represent database schema.
Reading between the lines, perhaps you are looking for something not covered by a web framework? Django's features are for responding to HTTP requests, managing a relational database, auth, email, admin, templates etc. If you're trying to do something not part of this, use other parts of the Python language. Django is a library; Python is the more fundamental tool used to build applications.
Finally, in my opinion, the best reason to not use Django is not the project itself (because it will do the job in 99% case), it's because all you learn is tied to Django.
Having learn Pydantic recently was a breed of fresh air, and I would reuse it in lots of projects, not only web projects.
