Recommendations for custom live view upload validation

I tried to implement my own entry validation, as described above, by extending the handle_info/3 that reacts to form changes.

See Uploads — Phoenix LiveView v1.2.5

But now I bumped into the issue: After evaluating the affected entries and setting appropriate errors, I am not allowed to use re-assign/3 the :upload key because it is a “reserved” one:

** (ArgumentError) :uploads is a reserved assign by LiveView and it cannot be set directly
    (phoenix_live_view 1.0.0-rc.6) lib/phoenix_component.ex:1324: Phoenix.Component.validate_assign_key!/1
    (phoenix_live_view 1.0.0-rc.6) lib/phoenix_component.ex:1275: Phoenix.Component.assign/3

So my initial reaction proved somewhat right: The LiveView authors do not want me to prod around in those internals.

But: What can I do now? What seems to be missing is an option for allow_uploads/3 for passing a custom validator function.

Ok, I got it working by not using assign/3 which is a bit frightening. For anyone daring enough, here is the implementation.

My customized handle_event/3 looks like this.

def handle_event(
      "validate_selected_images",
      _params,
      %{
        assigns: %{
          uploads: %{images: selected_images} = uploads,
          existing_files: existing_files
        }
      } =
        socket
    ) do

  existing_names =
    Enum.map(existing_files, fn entry ->
      entry.file_name
    end)

  checked_images = mark_duplicate_names(selected_images, existing_names)

  assigns = Map.put(socket.assigns, :uploads, Map.put(uploads, :images, checked_images))

  {
    :noreply,
    Map.put(socket, :assigns, assigns)
  }
end

Here existing_files is a list containing application specific metadata structs. As you can see I use Map.put/3 to hack my way around the “reserved assign” restriction.

The function mark_duplicate_names/2 looks like this:

defp mark_duplicate_names(%Phoenix.LiveView.UploadConfig{} = upload, existing_names) do
  processed_entries =
    upload.entries
    |> Enum.map(fn %Phoenix.LiveView.UploadEntry{} =
                      entry ->
      if entry.client_name in existing_names do
        Map.put(entry, :valid?, false)
      else
        entry
      end
    end)

  upload
  |> Map.put(:entries, processed_entries) # replace file entries
  |> Map.put(
    :errors,
    Enum.map(processed_entries, fn %Phoenix.LiveView.UploadEntry{} = entry ->
      if entry.valid? do
        nil
      else
        {entry.ref, :name_duplicate}
      end
    end)
    |> Enum.reject(fn val -> is_nil(val) end) # create a list of error tuples for all entries with error
  )
end

Finally I implement a fourth error_to_string/1 function:

defp error_to_string(:name_duplicate), do: "A file with this name already exists"

Small update: In order to preserve the errors that got returned by Phoenix’ standard evaluation (size, type, upload count issues), I had to modify the function slightly.

defp mark_duplicate_names(%Phoenix.LiveView.UploadConfig{} = upload, existing_names) do
  processed_entries =
    upload.entries
    |> Enum.map(fn %Phoenix.LiveView.UploadEntry{} =
                      entry ->
      if entry.client_name in existing_names do
        Map.put(entry, :valid?, false)
      else
        entry
      end
    end)

  duplicate_name_errors =
    Enum.map(processed_entries, fn %Phoenix.LiveView.UploadEntry{} = entry ->
      previous_error? = Enum.any?(upload.errors, fn {ref, _msg} -> ref == entry.ref end)

      if entry.valid? or previous_error? do
        nil
      else
        {entry.ref, :name_duplicate}
      end
    end)
    |> Enum.reject(fn val -> is_nil(val) end)

  # Combine existing errors with additional ones evaluated here.
  errors = upload.errors ++ duplicate_name_errors

  upload
  |> Map.put(:entries, processed_entries)
  |> Map.put(:errors, errors)
end