This is a drive-by change after spending some quality time with the
mount code. The `id` field of `Mount` is calculated only once and
only when creating a `Mount`. This seems slightly dangerous as
any change to an attribute after creation will not update the
id. This means two options:
1. dynamically update the `id` on changes
2. forbid changes after the `id` is calculcated
I went with (2) but happy to discuss of course but it seems more
the spirit of the class.
It also does the same change for "devices.Device"
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