Fat Models – A Django Code Organization Strategy | Hacker News Reader

Fat Models – A Django Code Organization Strategy

redbeacon.github.io

52 pointsbit2pixel32 comments

Fat models are slightly better than having that crud in your views, but they're still an antipattern. Models should describe the relationships between entities in your business, not be the junk drawer for stuff that doesn't go anywhere else.

Fat models are already a code smell, but passing the request object to your model should really set off alarm bells. You've now made it extremely messy to use your models outside of a web context, making it impossible to unit test them, and pretty much wiped out the purpose of the controller as an encapsulation layer.

Domain-driven design (DDD) provides a lot of patterns that help to keep from either having model code in your views, or these types of junk drawer models.

Basically, DDD as it applies to MVC webapps is: If it's a complex query of some sort, create a query repository that returns model objects rather than a "fat" static method on the model itself. If it's a business transaction, create logic objects that encapsulate the biz logic, and service objects that applies the logic to the models. The controller serves as the broker between requests, services, and entities. You end up with much more testable and reusable code this way.

The article precisely says you should not pass the request to the models.

That doesn't make it any less of an antipattern. The "fat model" pattern is essentially "if you can't figure out where it goes, put it in the model." In the article's example, the credit card processing code is wrapped up into the quote model. I can't think of a single situation in which this is a good idea.

The "fat model" pattern is simply organising your logic and relationships into the model layer. How well you organise it is up to you. If you just dump things from the controllers into random parts of the model then that's your choice but there are plenty of ways to design it well.

I'm closely involved in a big system that uses Catalyst (perl's nearest equivalent to Django). We spend allmost all of our time in Model classes (MyDomain::Model::Whatever, not MyWebApp::Model::Whatever that's just a dumb connector). It works well and I get the shits every time I have to deal with code that doesn't separate this stuff out properly. Also works on the front end using modern javascript.

I have most of my logic in the views (because I didn't now any better when starting my project). I know the Django way suggests fat models (well I know that now). But is it honestly that big of a deal? I know in my app that any complex logic will be in the views. What will I gain by following these best practices?

Yeah, try having a fat controller or view, on a rapidly developing codebase with poor specs which change mid-project regularly while avoiding getting fired.

Sounds very like my work.

Yeah, schema changes are a pain in the arse, but I don't see that moving my code from views to models would have a great deal of impact (though I am gradually moving the code that way when it does need changed).

A better answer is, that you get to decouple the functionality of your application's business logic from the web portion. This is achieved trivially purely by applying disciplined coding standards. Removing the web dependency makes the code much much easier to write automated tests for, and it gives a sane development path when making minor or major changes to feature sets.

But the article passes other unrelated things to the models.

If it's a complex query of some sort, create a query repository that returns model objects rather than a "fat" static method on the model itself.

I believe that the "Django way" of doing that is to have a custom "Manager" class;

https://docs.djangoproject.com/en/1.6/topics/db/managers/#cu... http://zmsmith.com/2010/04/using-custom-django-querysets/

"the junk drawer for stuff that doesn't go anywhere else"

I really want to know where to put that stuff. Creating an object to put the "filterBlah" function that you only use once is overkill, so it sits there as a one-off method in your View or Controller or Model. I've worked with large enough code bases for long enough that I know there's always hacky junk there, and I'd just like to know more strategies for addressing it. Do large code bases have a strategy for putting the one-offs in a place where they are obvious and organizable?

I'm now working on a 5M line code base, and at that scale, you kind of have to put the code into acceptable, bad, worse and awful categories and come up with strategies to fix it. There IS no good code at this point. I've actually done refactoring with regexes, but I want better tools for reasoning about extremely large and weird code bases.

In rails filter functions like that are kept in the model. They're composable and lazily evaluated so you don't really end up with giant, one-off methods. In python I'd expect to find this sort of functionality in a service class (with each domain object having it's own service class).

you could simply use unbound functions which you put into a Python module which fits the bill. For example you could create a file services.py - or something more related to your actual domain.

I write my code as if there will be multiple user interfaces. Usually there are not multiple user interfaces, but thinking as if there will be has a positive impact on the code. Thinking as if there will be multiple user interfaces means putting as little code as possible into the views, since there will be more than one. Some of those user interfaces might not even have Django views and templates, such as a service exposed via Zero MQ. This naturally leads me to putting as much logic as I can either into the models, or sometimes into other modules which in turn use the models. About the only code that goes into views then is the glue that looks things up and gets things ready to be passed into a model.

I can only shake my head in disbelief when I heard Rails or Django developers touting their framework and saw their actual codebase (ever see FatCRM codebase back in the days? it has been improved significantly these days, but it used to be ... hard to read).

I even shake my head faster when I read the split in opinions in Rails world: the Rails DHH and the DCI/Service Rails.

Moreover, I keep shaking my head when the Django and Rails community voiced their complain over their so-called bloated framework just because they want an API service.

Last but not least, I shake my head again whenever I heard that Django and Rails is not like NodeJS.

C'mon guys... you guys speak a lot of bad things about JAVA yet you've become Java Enterprise Edition.

I suppose JavaEE (especially 6 and 7) does work better in terms of writing maintainable code.

Stuff like Service pattern, Repository pattern, Data mapping pattern, DTO, Value Objects, Mocking are pretty much "common" in most Java projects I've seen or worked on.

I felt that no matter how cool Rails and Django are, when it comes to medium-to-large codebase, Rails/Django are definitely either following the patterns that pretty much common in JavaEE or may not be so comfortable to use anymore.

I honestly think this guy doesn't get it.

The proper way to separate concerns in Django is through applications, which have their own URL routes, models, and views (although they can invoke any other thing in another app).

Through Class-based views you can define base views for common behavior for templates and basic logic, and the built-in views are usually good enough to contain most patterns and easy enough to customize for any other use case you might think of.

A good rule of thumb is that an application should generally have less than a dozen views (I'd say 5 or 6 is enough already). Since each app has its own forms and models file, adding utility methods in an extra file contained within an app is reasonable.

I've used this to manage Django apps with over 50.000 lines of custome code (not counting several dozen extra apps and admin sites with their own customization) and it has never been a problem.

Fat models are just a massive problem. If Python supported extension methods then perhaps you could load your own extensions when performing an action, but a proper design of forms and views that aims for maximum simplity has never failed me.

I would stick with no classes being "fat". Breaking up fat classes into many smaller classes leads to code being really easy to understand, test, and develop. I tend to stick with the Single-Responsibility-Principle and classes do not become unmanageable. Some people argue that many small classes leads to complex code, but I have not found that to be true in any case I have come across.

Ah, but there be monsters in that sea too. I used to subscribe to the many-focused-models camp, but depending on the relationships you're trying to model, that can become damn-near unmaintainable as well.

I've actually begun building out more "wide" models, that is, models with a great many fields set to allow null/blank, because mixins and abstract base classes are very hard to maintain 3-4 years out. Don't misunderstand me, there's a place for those, but it's easy to get lost on road to abstraction.

Of course, the lesson is that if you're representing complex relationships in code, the implementation is going to wind up complex to some extent.

There's another option besides mixins and base classes, composition. You can break your fat model into lots of smaller classes that you aggregate into your original model. The big advantage over using mixins is that the scope is kept much smaller. Keeping scopes small in general is always a good way to scale up to a large codebase (functional languages get this pretty much for free).

Yes, I generally stick to composition. "High level" classes end up using a bunch helper classes to perform specialized tasks. I find that I do not often find the need for an abstract base class or mixins, but I will use them if it makes sense.

"Fat" in this case just means they contain business logic. Many people use their ORM models _exclusively_ as a data layer with no business logic living in the object at all.

Which, if you're following the SRP, is the right thing to do, no?

It depends, is it anemic model? or ActiveRecord like model?

Not sure I agree with fat models being a "rarely seen, alternative code organization strategy".

It's a natural state which most django codebases tend towards. There's a natural Views use ModelForms use Models data flow which leads to bloat somewhere in that chain as you add code. Fat models are easiest to test and the natural place to put most things, but eventually massive models get unwieldly and you start putting things into helpers. The core thing the author seems to have conflate util functions that have access to a request and helpers that have no connection to the request/response cycle.

In my experience, fat models are the most common approach to organising code in Django projects. As another comment stated, it's better than having fat views, but it's not ideal.

"Fat" abstract models is the way to go if you have logic that operates on the data. It's not even fat. There were fields assigned values in the example. That is exactly what model methods should do.

"Fat" abstract forms is the way to go if your forms have crispy layouts, placeholder initializers, or other stuff that that directly has to do with forms.

Views can be easily split into categorical files and shouldn't contain much logic other than bringing models and presentation together.

Sounds like thinking about what you're doing is the way to go to not have code get out of hand.

Maybe you guys can help me apply this to an actual Django project I'm working on.

So I made an "Order" model to track orders. But now I have to add a lot of logic regarding things such as what can be ordered together, and which users can order what.

Where you keep that logic? It seems to make sense to me to put it right in the model, no?

Personally I would make models a package rather than just a module (since you sy you have a lot of order related logic). I would create one or more modules in this package, presumably one of those for orders.

Then I would actually create the base logic which clearly belongs to one Order instance as methods on the Order class.

If you have something like "can x and y and (...)" be ordered together I would make an unbound function in your orders module which takes an iterable (Lists etc.) of Orders and works on it.

Note that I am by no means a Django guru or something like that.

There is no "one right way", but when thinking about code organization and architecture I find inspiration from the "clean architecture"[1] and similar approaches. The dependency rule can really turn your world upside down!

[1] http://blog.8thlight.com/uncle-bob/2012/08/13/the-clean-arch...

Does anyone else feel weird knowing that when you call that model method it may or may not call out to a background worker task?

Why not just throw all of your celery tasks that are related to an app inside of a worker.py file or something else?

If you import from service.py or worker.py then it becomes obvious the thing you're doing is happening in the background.

Is this article pointing out the most common way of organizing code in Django as an advice?

I'm such a failure at marketing ... sigh

Reply on news.ycombinator.com