A user's preferences for a view — which columns a table shows and in what order, and whatever else the view's owner keeps (a sort, filters) — one JSON object per (user, key).

key names the view and is namespaced by its owner: "users", "website_access.attempts", "catalogue.detail_items", "crm.role.<uuid>". Each top-level field of the object is written whole: put/3 patches the fields it is given with prefs || changes inside the upsert, so two tabs changing different fields both keep their change, and nothing is rebuilt from a copy of the user a page happens to hold. delete_fields/3 takes a field back out — which is what "reset" means: the view goes back to following its default.

PhoenixKitWeb.TableColumns is the table-columns layer on top; core gives meaning only to the "columns" field.

A user is a %{uuid: _} or a uuid; nil (nobody signed in) reads as no preferences and writes {:error, :no_user}. Nothing here raises: a read that fails is no preferences, a write that fails answers {:error, _}.