test: override stale distance_km when filling a missing pos

Makes the 'always recompute' intent explicit by removing the dead-code
short-circuit in add_distance_change — the candidate query already
requires at least one nil pos, so any previously stored distance is
tied to a stale grid/pos pair and must be overwritten.

lib/microwaveprop/workers/contact_position_backfill_worker.ex

@@ -78,23 +78,20 @@ defmodule Microwaveprop.Workers.ContactPositionBackfillWorker do
   defp add_pos_change(updates, _field, _existing, nil), do: updates
   defp add_pos_change(updates, field, _nil, new), do: [{field, new} | updates]
 
-  defp add_distance_change(updates, contact, pos1, pos2) do
-    cond do
-      pos1 == nil or pos2 == nil ->
-        updates
+  # Always overwrite distance_km when both positions resolve — the candidate
+  # query only selects contacts with at least one nil pos, so any previously
+  # stored distance is tied to a stale grid/pos pair.
+  defp add_distance_change(updates, _contact, nil, _pos2), do: updates
+  defp add_distance_change(updates, _contact, _pos1, nil), do: updates
 
-      not is_nil(contact.distance_km) and contact.pos1 != nil and contact.pos2 != nil ->
-        updates
+  defp add_distance_change(updates, _contact, pos1, pos2) do
+    distance =
+      pos1["lat"]
+      |> Radio.haversine_km(pos1["lon"], pos2["lat"], pos2["lon"])
+      |> round()
+      |> Decimal.new()
 
-      true ->
-        distance =
-          pos1["lat"]
-          |> Radio.haversine_km(pos1["lon"], pos2["lat"], pos2["lon"])
-          |> round()
-          |> Decimal.new()
-
-        [{:distance_km, distance} | updates]
-    end
+    [{:distance_km, distance} | updates]
   end
 
   defp maybe_touch_updated_at([]), do: []

test/microwaveprop/workers/contact_position_backfill_worker_test.exs

@@ -136,5 +136,34 @@ defmodule Microwaveprop.Workers.ContactPositionBackfillWorkerTest do
       distance = Decimal.to_integer(reloaded.distance_km)
       assert distance > 0 and distance < 1000
     end
+
+    # Scenario: user updates grid1 to higher precision via direct DB write and
+    # nulls pos1 but leaves distance_km at the old (less-precise) value. Worker
+    # must re-resolve pos1 AND overwrite the stale distance_km.
+    test "overrides stale distance_km when filling a missing pos" do
+      pos2 = %{"lat" => 30.3, "lon" => -97.7}
+      stale_distance = Decimal.new(9999)
+
+      contact =
+        insert_raw_contact!(%{
+          grid1: "EM12",
+          grid2: "EM00",
+          pos1: nil,
+          pos2: pos2,
+          distance_km: stale_distance
+        })
+
+      assert {:ok, %{updated: 1}} = ContactPositionBackfillWorker.perform(%Oban.Job{args: %{}})
+
+      reloaded = Repo.reload!(contact)
+      assert %{"lat" => _, "lon" => _} = reloaded.pos1
+
+      refute Decimal.equal?(reloaded.distance_km, stale_distance),
+             "expected distance_km to be recomputed from refined grid, got #{reloaded.distance_km}"
+
+      # EM12 <-> EM00 is ~333 km; sanity check the new value is in that ballpark.
+      new_distance = Decimal.to_integer(reloaded.distance_km)
+      assert new_distance > 0 and new_distance < 1000
+    end
   end
 end