Two-phase index rewrite for drag-to-reorder list views.

Given a list of UUIDs in their new display order and an Ecto schema, rewrites a position field on the matching rows to 1..N matching the order of the input list. The write runs in two passes inside a transaction — first to negative indices, then to positive — so a unique index on the position column (should one ever be added) wouldn't trip mid-update.

Consumers that need pre-write validation (scope checks, permission guards) or post-write side effects (activity logging, PubSub broadcasts) should wrap this helper rather than fold the logic in here. PhoenixKitProjects.reorder_projects/2 is the reference consumer doing exactly that — this module owns only the index-rewrite primitive.

Rows are matched on uuid by default; pass key: :id (any field) for a schema keyed some other way — an integer primary key, a slug. The payload's ids are cast with that field's Ecto type, so the strings a drag hook sends ("42") match an integer column.

Entries that are not a valid key (a non-UUID for :uuid, a non-integer for an integer key) are silently filtered — a stale or malformed drop event can't poison the rewrite. Duplicates dedup last-write-wins via Enum.uniq/1.

Example

defmodule MyApp.Endpoints do
  alias PhoenixKit.Utils.Reorder

  def reorder_endpoints(ordered_ids) do
    Reorder.reorder(MyApp.Endpoint, ordered_ids, :sort_order, repo: repo())
  end
end