Cached collections only work if there is one template

[tenderlove]

I don't know this is the right fix necessarily.

When rendering a collection, each object in the collection can respond to to_partial_path. The template renderer will render the partial returned by that method. However, each object in the collection could return a different template. That means there isn't one template associated with the collection. Our collection template cache code depends on the collection using just one template. If you try to set cached: true on a collection that doesn't have just one template associated with it, it will raise an exception.

The patch in this PR fixes the exception, but doesn't actually cache the templates. Obviously, this use case isn't supported today, and it seems like nobody has noticed.

1. I'm not sure how or if we should support caching collections like this
2. If we decide not to support this, should it raise an exception?

[rafaelfranca]

I can't think in a way we can cache collections like this so I feel like raising an exception on this case is better than silently not caching it. At least the users can make the decision to use the same template if cache is important or remove the cache call that was not doing anything.

[tenderlove]

I can't think in a way we can cache collections like this so I feel like raising an exception on this case is better than silently not caching it. At least the users can make the decision to use the same template if cache is important or remove the cache call that was not doing anything.

Cool. That was my feeling too, but I wanted to hear other opinions. I'll update this to raise a NotImplementedError.