feat(accounts): derive 10-char home_grid from explicit lat/lon

lib/microwaveprop/accounts/user.ex

@@ -220,9 +220,10 @@ defmodule Microwaveprop.Accounts.User do
 
   Accepts either a Maidenhead grid or an explicit lat/lon. When only the
   grid is given, lat/lon are derived from its centre; when only lat/lon
-  are given, the grid is left untouched. An empty `home_grid` paired
+  are given, a 10-character grid is computed from the coordinate pair so
-  with explicit nil lat/lon clears all home fields (used by the "clear
+  callers always end up with both fields populated. An empty `home_grid`
-  home" affordance on /rover).
+  paired with explicit nil lat/lon clears all home fields (used by the
+  "clear home" affordance on /rover).
   """
   @spec change_home_qth(t() | Ecto.Changeset.t(), map()) :: Ecto.Changeset.t()
   def change_home_qth(user, attrs) do
@@ -231,6 +232,7 @@ defmodule Microwaveprop.Accounts.User do
     |> normalize_home_grid()
     |> validate_home_grid_format()
     |> derive_home_latlon_from_grid()
+    |> derive_home_grid_from_latlon()
     |> validate_home_latlon_range()
     |> require_home_qth_pair()
   end
@@ -294,6 +296,30 @@ defmodule Microwaveprop.Accounts.User do
     end
   end
 
+  # If lat/lon were provided without a grid, encode a 10-char Maidenhead
+  # grid from the coordinates so the UI always has a human-readable label
+  # to render and downstream callers can rely on `home_grid` being set
+  # whenever the QTH is set. Skipped on invalid changesets (lat/lon range
+  # check has not run yet) and when both fields aren't numeric.
+  defp derive_home_grid_from_latlon(changeset) do
+    lat = get_field(changeset, :home_lat)
+    lon = get_field(changeset, :home_lon)
+
+    cond do
+      not changeset.valid? ->
+        changeset
+
+      not (is_number(lat) and is_number(lon)) ->
+        changeset
+
+      get_field(changeset, :home_grid) not in [nil, ""] ->
+        changeset
+
+      true ->
+        put_change(changeset, :home_grid, Maidenhead.from_latlon(lat * 1.0, lon * 1.0, 10))
+    end
+  end
+
   defp validate_home_latlon_range(changeset) do
     changeset
     |> validate_number(:home_lat, greater_than_or_equal_to: -90.0, less_than_or_equal_to: 90.0)

test/microwaveprop/accounts/user_test.exs

@@ -461,6 +461,27 @@ defmodule Microwaveprop.Accounts.UserTest do
       assert Ecto.Changeset.get_change(changeset, :home_lon) == -96.7
     end
 
+    test "derives a 10-char home_grid when lat/lon are given without one" do
+      changeset = User.change_home_qth(%User{}, %{"home_lat" => 33.10, "home_lon" => -96.625})
+      assert changeset.valid?
+      grid = Ecto.Changeset.get_change(changeset, :home_grid)
+      assert is_binary(grid)
+      assert String.length(grid) == 10
+      assert String.starts_with?(grid, "EM13qc")
+    end
+
+    test "preserves an explicit grid when both grid and lat/lon are provided" do
+      changeset =
+        User.change_home_qth(%User{}, %{
+          "home_grid" => "EM13qc",
+          "home_lat" => 33.10,
+          "home_lon" => -96.625
+        })
+
+      assert changeset.valid?
+      assert Ecto.Changeset.get_change(changeset, :home_grid) == "EM13qc"
+    end
+
     test "rejects invalid grids" do
       changeset = User.change_home_qth(%User{}, %{"home_grid" => "ZZ99zz"})
       refute changeset.valid?