feat: optionally attach a coverage to a device at the chosen site

After picking a site in the coverage form, a new optional dropdown
lets the user attach the coverage to one of the devices at that site.
The dropdown reloads on every site change. If the site has no devices
yet, a hint replaces it.

Schema: nullable device_id FK on coverages (on_delete: nilify_all so
deleting a device leaves the coverage standing). Filtered by both
site_id and organization_id when loading the dropdown options.

Show page lists the attached device by name (or IP) when set.

CHANGELOG.txt

@@ -1,3 +1,14 @@
+2026-05-06
+feat: optionally attach a coverage to a specific device at the chosen site
+  Schema: added nullable `device_id` FK to coverages (migration
+  20260506194539_add_device_id_to_coverages, on_delete :nilify_all).
+  Form: after picking a site, a "Attach to device (optional)" dropdown
+  appears with that site's devices (filtered to the current org). The
+  dropdown reloads on every site change. If the site has no devices the
+  hint "This site has no devices yet" replaces the dropdown.
+  Show page: when set, the attached device's name (or IP) is displayed
+  in the parameters panel.
+
 2026-05-06
 refactor: imperial coverage form + Deygout/curvature propagation, drop UI clutter
   Form (lib/towerops_web/live/coverage_live/form.html.heex):

lib/towerops/coverages/coverage.ex

@@ -16,6 +16,7 @@ defmodule Towerops.Coverages.Coverage do
 
   alias Ecto.Association.NotLoaded
   alias Towerops.Coverages.Antenna
+  alias Towerops.Devices.Device
   alias Towerops.Organizations.Organization
   alias Towerops.Sites.Site
 
@@ -73,6 +74,7 @@ defmodule Towerops.Coverages.Coverage do
 
     belongs_to :site, Site
     belongs_to :organization, Organization
+    belongs_to :device, Device
 
     timestamps(type: :utc_datetime)
   end
@@ -111,6 +113,8 @@ defmodule Towerops.Coverages.Coverage do
           site: NotLoaded.t() | Site.t() | nil,
           organization_id: Ecto.UUID.t() | nil,
           organization: NotLoaded.t() | Organization.t() | nil,
+          device_id: Ecto.UUID.t() | nil,
+          device: NotLoaded.t() | Device.t() | nil,
           inserted_at: DateTime.t() | nil,
           updated_at: DateTime.t() | nil
         }
@@ -125,7 +129,7 @@ defmodule Towerops.Coverages.Coverage do
     tx_clearance_m foliage_tuning
     latitude_override longitude_override
     radius_m cell_size_m receiver_height_m rx_threshold_dbm
-    status progress_pct site_id
+    status progress_pct site_id device_id
     height_agl_ft height_above_rooftop_ft receiver_height_ft tx_clearance_ft
     radius_mi frequency_ghz
   )a
@@ -189,6 +193,7 @@ defmodule Towerops.Coverages.Coverage do
     |> validate_inclusion(:status, @statuses)
     |> validate_pixel_budget()
     |> validate_antenna_exists()
+    |> foreign_key_constraint(:device_id)
     |> foreign_key_constraint(:site_id)
     |> foreign_key_constraint(:organization_id)
     |> unique_constraint(:name,

lib/towerops_web/live/coverage_live/form.ex

@@ -5,6 +5,7 @@ defmodule ToweropsWeb.CoverageLive.Form do
   alias Towerops.Coverages
   alias Towerops.Coverages.Antenna
   alias Towerops.Coverages.Coverage
+  alias Towerops.Devices
   alias Towerops.Sites
 
   # Defaults expressed in the user's preferred units (imperial / GHz).
@@ -26,7 +27,8 @@ defmodule ToweropsWeb.CoverageLive.Form do
      |> assign(:organization, organization)
      |> assign(:sites, sites)
      |> assign(:antennas, antennas)
-     |> assign(:antenna_options, antenna_options(antennas))}
+     |> assign(:antenna_options, antenna_options(antennas))
+     |> assign(:devices, [])}
   end
 
   @impl true
@@ -66,6 +68,7 @@ defmodule ToweropsWeb.CoverageLive.Form do
     socket
     |> assign(:page_title, t("Edit Coverage"))
     |> assign(:coverage, coverage)
+    |> assign(:devices, devices_for_site(socket.assigns.organization.id, coverage.site_id))
     |> assign_form(Coverages.change_coverage(coverage))
   end
 
@@ -76,6 +79,9 @@ defmodule ToweropsWeb.CoverageLive.Form do
       |> Coverages.change_coverage(attrs)
       |> Map.put(:action, :validate)
 
+    new_site_id = Map.get(attrs, "site_id")
+    socket = maybe_refresh_devices(socket, new_site_id)
+
     {:noreply, assign_form(socket, changeset)}
   end
 
@@ -170,6 +176,44 @@ defmodule ToweropsWeb.CoverageLive.Form do
     end)
   end
 
+  # Reload the devices dropdown when the user picks a different site.
+  # Clearing site_id clears the dropdown.
+  defp maybe_refresh_devices(socket, site_id) do
+    current_site_id =
+      case socket.assigns[:form] do
+        nil -> nil
+        form -> form[:site_id].value
+      end
+
+    if to_string(site_id) == to_string(current_site_id) do
+      socket
+    else
+      assign(socket, :devices, devices_for_site(socket.assigns.organization.id, site_id))
+    end
+  end
+
+  defp devices_for_site(_org_id, site_id) when site_id in [nil, ""], do: []
+
+  defp devices_for_site(org_id, site_id) do
+    site_id
+    |> Devices.list_site_devices()
+    |> Enum.filter(&(&1.organization_id == org_id))
+  end
+
+  @doc false
+  def device_label(device) do
+    case device do
+      %{name: name, ip_address: ip} when is_binary(name) and name != "" ->
+        "#{name} (#{ip})"
+
+      %{ip_address: ip} ->
+        "#{ip}"
+
+      _ ->
+        ""
+    end
+  end
+
   defp populate_imperial_virtuals(%Coverage{} = c) do
     %{
       c

lib/towerops_web/live/coverage_live/form.html.heex

@@ -57,6 +57,21 @@
           required
         />
 
+        <.input
+          :if={@devices != []}
+          field={@form[:device_id]}
+          type="select"
+          label={t("Attach to device (optional)")}
+          prompt={t("No specific device")}
+          options={Enum.map(@devices, &{device_label(&1), &1.id})}
+        />
+        <p
+          :if={@form[:site_id].value not in [nil, ""] and @devices == []}
+          class="text-xs text-gray-500 dark:text-gray-400"
+        >
+          {t("This site has no devices yet — coverage will not be attached to a specific device.")}
+        </p>
+
         <.input
           field={@form[:antenna_slug]}
           type="select"

lib/towerops_web/live/coverage_live/show.ex

@@ -9,7 +9,7 @@ defmodule ToweropsWeb.CoverageLive.Show do
   @impl true
   def mount(%{"id" => id}, _session, socket) do
     organization = socket.assigns.current_scope.organization
-    coverage = Coverages.get_coverage!(organization.id, id)
+    coverage = organization.id |> Coverages.get_coverage!(id) |> Towerops.Repo.preload(:device)
     antenna = Antenna.get(coverage.antenna_slug)
     eirp = Coverage.eirp_dbm(coverage, (antenna && antenna.gain_dbi) || 0.0)
 

lib/towerops_web/live/coverage_live/show.html.heex

@@ -197,6 +197,13 @@
             label={t("Antenna")}
             value={(@antenna && @antenna.model) || @coverage.antenna_slug}
           />
+          <.param_row
+            :if={
+              @coverage.device_id and Ecto.assoc_loaded?(@coverage.device) and @coverage.device
+            }
+            label={t("Device")}
+            value={@coverage.device.name || @coverage.device.ip_address}
+          />
           <.param_row :if={@antenna} label={t("Manufacturer")} value={@antenna.manufacturer} />
           <.param_row :if={@antenna} label={t("Antenna gain")} value={"#{@antenna.gain_dbi} dBi"} />
           <.param_row label={t("Frequency")} value={format_ghz(@coverage.frequency_mhz)} />

priv/repo/migrations/20260506194539_add_device_id_to_coverages.exs

@@ -0,0 +1,14 @@
+defmodule Towerops.Repo.Migrations.AddDeviceIdToCoverages do
+  use Ecto.Migration
+
+  def change do
+    alter table(:coverages) do
+      # Optional FK to a device at the parent site. The coverage represents
+      # the radio mounted on this device's tower. nil means "no specific
+      # device" (e.g. a planning study).
+      add :device_id, references(:devices, type: :binary_id, on_delete: :nilify_all)
+    end
+
+    create index(:coverages, [:device_id])
+  end
+end