There's an important tradeoff being discussed here, which is, "can we get directly the data we want from the query", versus, "do we need to load all the data into our app first and filter it there". This is of course the critical thing that a lot more people need to learn, and the work I do with SQLAlchemy is all about this. But in the SQLA approach, we use Python constructs on the app side which expand into SQL functions when rendered in a query. The effect is very similar to that which I see in most of the examples in your posts.
While I think advanced data models and rich SQL-side functionality are essential, the usage of stored procedures is IMHO not the only way to get there. In practice I often use a mix of both, depending on how verbose the function needs to be.
Keeping SQL functions as app-side constructs has the advantage of source code management. It's easier to support multiple kinds of backends (I run against PG and SQL Server a lot) since you aren't tied to a stored procedure language. There's no need to emit new stored procedure definitions to the database in order to support new features of the application. You don't have the issue of updating a stored procedure on the database side such that multiple application versions, targeted to different versions of the database function, still continue to function. I think there are ways to approach these problems in favor of SPs, but they require some thought on how the source code is maintained, managed, and deployed. For now I've just stuck with keeping most SQL functions on the app side.
The big namespacing problems I see are, what if two different kinds of "classes" want to have the same method name ? The definition of a PG function here creates a name that's global to the whole schema - this suggests we may want names that are qualified with a "class name". And what if you do in fact need two versions of the same function present to support different application versions ? In that case maybe we want to qualify the names of the functions with version ids as well. This actually sets up a great opportunity to use an application side system of rendering class/version qualified SQL names in response to plain names on the app side.
I guess my point is that the "app logic in stored procedures" approach is interesting, it has some management/deployment issues that also might be interesting to solve, but app-rendered SQL when using an effective enough app-side toolkit can solve the problem just as well in most cases.
