In only the trivial cases can you defer the actual SQL queries from being performed before the view is rendered.
Claiming that lazy loaded queries is only a benefit for "trival cases" is a strawman. It's a hugely powerful functionality for ActiveRecord that you can utilize in many ways, and would be very hard to implement without low level support.
Cached attributes can often easily be made available via concise single model methods that operate transparently without the controller OR the view needing to know they are cache-backed. Plus, even if you are loading stuff out of memcached in the controller, it's going to be fast, because that's the whole point of memcached.
ActiveRecord meanwhile, normally takes a huge percentage of rendering time. Being able to defer those queries while still allowing the controller to declare them is actually a huge combination of performance flexibility and separation of concerns. Previously, if you wanted to defer them "cleanly", you'd have to create model methods, but even there you would have to pass params through somehow or generally do something uglier than what you have to do now.
1.The performance goal is to return http & html headers as quickly as possible.
2. Business logic belongs in the models, as triggered by method invocations from the controller.
3. A good deal of time is spent on business logic and request handling (authentication / set-up / before_filter stuff.) Often, this requires network I/O to backend systems (or databases.) While some data retrieval is necessary only to render the view (and can thus be deferred gracefully,) other times your application logic depends the completion of these lengthy requests in order to complete the desired state modification.
4. Since we want to return the headers as quickly as possible, we can either a) figure out how to send the headers before the controller or b) figure out how to delay the processing until after the controller.
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I think that that a) is better than b).
I like views that exclusively take data and format it for output. I like having the core business logic in the models, and I like having the system guards and request setup concentrated in the controllers. This way, I can have an exhaustive understanding of the tasks performed by an action without having to read through all of the views and their partials.
If we hide some of the processing within model actions and call those model actions from the view, then I no longer can assume that all major processing / I/O has happened by simply reading the controller and the model methods it invokes. Instead, I also have to read all of the views.
This violates the expectations I have about processing times and MVC.
If instead, we could detect the requested format and return immediately with the headers, then we could perform the overwhelming majority of the processing while the browser is busy downloading static assets.
However I see the core team's point that they don't want to completely overhaul the API in a way that's going to break arguably a majority of existing apps, and at the same require the developer to pay attention to more details than may be necessary.
However on the topic of breaking expectations about processing times, I couldn't disagree more. To me, the main benefit of MVC (or any architecture choice) is in isolating responsibilities, not isolating code execution. It's not a big mental leap to conceive of a ActiveRecord relation as declaring some data that's needed that may or may not be actually queried, depending on if the view needs it. This helps your MVC separation, because it means you don't need to extract view logic from your template just to make sure the controller doesn't load something unnecessarily. Lazy execution of this form is a very powerful optimization technique, just ask any Haskeller.
(not really, boom, hat trick: http://img.skitch.com/20100908-tpruqq1pqsw3rsyh2ggn5qb7ac.pn...)
I'll let the meme die now. Doing it in a before filter would have the same effect -- unnecessarily increasing the latency before the headers are sent to the web browser. Wether the time is spent in a before filter or a controller action, the key is that the time is being spent before the headers are delivered to the browser. If you are trying to minimize the time it takes to return the header, then you'll want to do that before almost anything else.
