{{-- Editable only while nothing references it. The code is not merely a foreign key: each host's DNS name was minted from it and exists at the provider, and the machine's own hostname was set from it while it was built. Renaming a code with hosts in it produces a datacenter called hel full of machines called fsn-01 — so once anything depends on it the field is a fact, and the reason is named rather than left for the operator to guess at. --}}