I'm not sure I see the point of having a separate `_slice_to_depth` method when its used this way; it would be a lot more efficient if `get_subclasses_recurse` just didn't recurse any further than one level in the first place on Django < 1.6. Which I think is how it worked previously. The new feature in this patch is really just a nice-to-have, not a critical improvement, so I'd really like to avoid any added inefficiency because of it.