Unfortunately here, Rails encourages putting each class into a separate file, so you have 10 classes spread over 10 files, which does increase complexity.
Having worked with both, there's a trade-off. Given that in Django you're (mostly) explicitly importing classes and modules rather than autoloading, it's handy to have them all in one place. OTOH, when your project grows, you end up with enormous model files (especially if you follow the fat models/thin views pattern). So you then have to split them into different apps, so fragmentation slips in eventually anyway. (In a rails project, unless you're bolting on engines and such, all your models are at least in one folder).
Where I definitely do prefer Django in this regard is that models declare their data fields, rather than them being in a completely different part of the source as in AR (not Mongoid, I now realise). Do I remember the exact spelling I gave to every column when I migrated them months ago? No. It's good to be able to see it all in one place rather than having an extra tab to cycle through. I don't see any practical benefit from decoupling here.
Especially since the Rails way is not "decoupling" in any real sense. Splitting tightly coupled code into multiple files != decoupling.
I also like that in Django, you declare the fields on the models first and then create the db migrations from them, rather than writing a db migration first to determine what fields the models have.
Indeed, decoupling is probably the wrong word here: I haven't seen an ORM implementation that was not tightly coupled to the database layer, which in the end is surely the point of an ORM - to represent stuff from the database in application code. (I know some people consider this a bad abstraction, but whatever.)
South/1.7 migrations is definitely the best way of the two to manage that coupling. Rails's charms lie elsewhere.
Right, and the debate raging in the Rails community now is whether your business logic should be in your models at all, or whether it should be extracted into plain old ruby objects, separating your domain model from your data model. Reason being, the OOP purists see it as a violation of the Single Responsibility Principle--an object should only have one reason to change, and the models are tightly coupled to the database schema so they have to change if the schema changes, plus you need to start up a database just to test their business logic, if you put business logic in them.
Meanwhile a lot of the practically minded developers like DHH just accept that their objects will be tightly coupled to the database and just deal with it, claiming that anything else would be adding unnecessary layers of indirection.
I am pretty new to Django, but I get the impression that it's not so hard to just not put your business logic in models.py, and put it in separate classes of plain old python objects instead. Maybe that's why I haven't heard about this debate playing out in the Django community the way it is in the RoR community...
Interesting. In CommonJS modules, a file can only export one thing. You could namespace multiple things into one exported object, though I find that granular dependencies can lead to insights about how reusable your modules really are.
If I'm in extreme mode I take the view that each file should be a single screen. That means a tangible reduction in the complexity of working on them (no more scrolling - each class is just in its own tab).
This can be solved with standard IDEs. Putting two modules or classes into a single file pretty much guarantees a level of coupling. This does not reduce complexity.
By that definition, I could just as easily argue that requiring different files for every class reduces cohesion. The idea that class definitions and file definitions are in any way related is a leaky abstraction.
I've never been a fan of the class-file coupling. It pulls me out of the mental model I'm trying to build in my head and forces me to think about file organization which is almost always inconsistent with the language semantics I'm dealing with.
I've used IDE's that make this more or less painful, but none that actually solved it. If anyone has any suggestions on one that does, I'd be interested to try it out. I don't really care what language. I can pick up enough to see what it feels like.
I also want to say that rich hickey talked about a file as a unit of code not being very good, but I don't recall where, or if he really said it. I want to say it was in a Datomic podcast right around when details about it were coming out.
I dislike having a class/module per file.