[mabcat]

Is there a good place or pattern for service methods in Django? I've got some very fat models right now, and it's a DRY improvement over having the fat in the views, but like you say it takes a lot of effort to trace what's going on.

[Alex3917]

Let's say you're following the approach of breaking down your project into separate apps, so you have an app called user_accounts. This app would be a folder containing files like:

  views.py, services.py, models.py, test_views.py

So in views.py you'd have a User class, with:

  A POST method that calls services.create_user(username, email_address, password)

  A GET method that calls services.get_user_profile(request.user)

  A PUT method that calls services.update_user_profile(x, y, z)

  A DELETE method that calls services.inactivate_user(request.user)

The return value of each of these views can just be whatever services.get_user_profile(request.user) returns, rendered into JSON.

Then each of the services performs whatever business logic it needs to, preferably directly in the method. But if it would be more readable split into multiple methods, then you can create some private helper methods in the services file prefixed with an underscore. You can also have a separate folder somewhere for utility functions meant to be reused across the app, e.g. get_user_emails(request.user, is_active=True, is_verified=True)

Basically though each view sanitizes the data, e.g. strips XSS out of strings, makes sure booleans are actually booleans, etc.

Then each service first does field-level validation with serializers, e.g. ensuring that usernames meet the appropriate requirements for usernames. Next if there is other business logic validation that needs to happen, it happens, e.g. making sure that only users with verified email addresses can perform certain actions.

After that you perform the actual business logic, e.g. transforming any data. Then you perform your CRUD operation, e.g. creating a user model. And lastly you return something, e.g. returning the user model.

Each endpoint and service method can be written pretty much following this pattern, which makes the codebase super readable because once you understand one endpoint you understand all of them. And the service methods are the reusable component of the architecture, so e.g. if you want the ability for admins to create users, then they are created with the exact same service method. (But called from your admin endpoints/services.)

[ralmeida]

That's almost exactly what I do, except for using a big monolithic app for the entire backend (called "core"), and making "services" a package with several modules inside.

It also resembles very much what I see in Java projects which use DDD (Domain Driven Design).

What's your take on the article's point that you should have fewer rather than more Django apps (citing the problem of inter-app-FKs)?

[Alex3917]

So my startup is built the way you describe, in terms of just having one main app, and I personally prefer this style.

The basic argument in favor of breaking down the Django project into multiple apps is that it makes the components more decoupled and reusable. But personally I think this is bullshit. If you want your apps to be reusable and decoupled then you need to put a ton of time into architecting them this way, the idea that you're going to get these benefits just from putting stuff into different folders is magical thinking. It seems like pretty much the textbook example of cargo cult programming.

That said for the client I'm currently working for, the decision was made to do it the 'standard Django way' in terms of breaking it into multiple apps. So far I haven't run into any issues here. I like it slightly less because I think having all the views in the views folder, and all the services in the services folder makes folks more likely to reuse code just by making it easier to find. But yeah, so far no real problems, but I'm also not expecting to see any magical benefits either.

[ralmeida]

I'm not aware of architectural patterns for service methods in Django (would like to find some as well), but what I did was to somewhat mimic a Java structure.

All the project is in one single app, which I unimaginatively called "core", and inside this app there's a "services" Python package (i.e. a folder with a __init__.py file inside). These have roughly one Python module (.py file) for each "category" of services. For example, there's thin layers like "user_service.py" (basically passes through to the relevant models), to more complex services like "dependency_x_integration_service.py", which connects to external service "X" and pulls some relevant data (say, user interaction datapoints), and bridges them to the models in the system.