Only by convention and common practice though. You can easily mix and match any part of the MVC as you see fit. This is where the "magic" comes in. Rails gives you a set of very sane, most-common-scenario conventions that are the basis for it's huge upfront productivity. Obviously once you step out of this convention zone you're going to hit a wall, because you'll actually have to write code. It just feels like a wall because everything that led up to it was so damn quick.
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Only by convention and common practice though. You can easily mix and match any part of the MVC as you see fit. This is where the "magic" comes in. Rails gives you a set of very sane, most-common-scenario conventions that are the basis for it's huge upfront productivity. Obviously once you step out of this convention zone you're going to hit a wall, because you'll actually have to write code. It just feels like a wall because everything that led up to it was so damn quick.