guardian/frontend — pull request #6409 by janua (2014-10-10 09:52:02+00:00)
Title: Facia-tool groups changes

We force `Trail` to have a group if they don't have a group. This is fine for when a `collection` does not have `groups`, as all `Trail`s will end up being sorted on 0. When the `Collection` then has `groups`, then the `Trails` will also have groups, this will still work correctly.

The problem is when we switch from a `Collection` with `groups` to no `groups`; the `group` on each `Trail` still lingers and suddenly the collection becomes unusable.

This will remove `group` from `Trail` if the containing `Collection` does not have groups.

@stephanfowler