How do I handle a nil value for `Plug.Upload` in params? [arc_ecto GitHub issue repost]

  • Elixir 1.2
  • Phoenix 1.2.1
  • arc: 0.6.0
  • arc_ecto 0.5.0

According to the Phoenix documentation, if no file is selected, Plug.Upload will not be present in the params map.

Finally, notice that when there is no data from the file input, we get neither the “photo” key nor a Plug.Upload struct. Here are the user_params from the log.

So if you submit a blank form, params[:user] will be nil. For example:

<%= form_for @changeset, @action, [multipart: true], fn f -> %>
  <%= file_input f, :avatar %>
  <%= submit "Submit" %>
<% end %>

Notice the absence of the “user” key.

%{"_csrf_token" => "...", "_method" => "put", "_utf8" => "✓"}

Which raises an error in the controller.

bad request to App.Account.AvatarController.update, no matching action clause to process request
  def update(conn, %{"user" => user_params}, current_user) do
    changeset = User.avatar_changeset(current_user, user_params)
    case Repo.update(changeset) do
      {:ok, _} ->
        redirect(conn, to: current_user_avatar_path(conn, :edit))
      {:error, changeset} ->
        render(conn, :edit, changeset: changeset, user: current_user)

We can hack around the error by including a hidden field in the form, forcing params[:user] to exist.

<%= hidden_input f, :id %>

But Plug.Upload will still be missing from the params. Which means the changeset will be valid. Notice changes being an empty map.

#Ecto.Changeset<action: nil, changes: %{}, errors: [], data: #App.User<>, valid?: true>

So, my questions are:

  1. What’s the point of |> validate_required([:avatar]) given that Plug.Upload is only present with file data? To reach this point in the changeset, a field must be present. It seems redundant then to validate for presence.
  2. How do we validate the presence of avatar?

Here’s my user schema:

defmodule App.User do
  use App.Web, :model
  use Arc.Ecto.Schema

  schema "users" do
    # ...
    field :avatar, App.AvatarUploader.Type

  def avatar_changeset(struct, params \\ %{}) do
    |> cast(params, [:avatar])
    |> cast_attachments(params, [:avatar])
    |> validate_required([:avatar])

Am I missing something?

1 Like

Well, you hit the thing I hate about validations in ORMs. You really do want to validate submitted form, and not validate the model, because of the reasons above. In my opinion, this really is the major design flaw, in Rails and by shared genes in Phoenix.

I tend to work around the issue by creating “Form objects” or “Form modules/structs” and handling validations outside of the “model” layer altogether. Which is more work, but has it’s rewards. Combination of and does the job for me at the moment. But this is radical approach, you may not want to take.

In your case, you can define 2nd function head in your controller, that will catch the nil. And it would call the current function you have, explicitly passing a nil to the avatar field, so the validations kick in properly.

1 Like

You can use ecto to do pure form validations without database tables. You event don’t need to define schema modules - you only need the changeset function. I use ecto frequently to cover that use case - what you’d usually call “form objects” in Rails. José talks about how to do this in the free Ecto 2.0 ebook, I’m not sure about examples in the wild.

As to files uploads in general, I succumbed a long time ago - there are always issues and it never works correctly. I decided to do direct uploads from the browser to some storage service like s3 or cloudinary, only sending the upload url back to backend. I never looked back. It removes a whole bunch of issues.


@michalmuskala thanks. Cloudinary looks great. Can you think of other options (libs or services) for doing direct uploads from the browser to S3 or similar? In Rails, I use Refile, which allows multiple direct uploads to S3. Arc doesn’t seem to have matured yet. There is another lib that’s inspired by Refile, but it doesn’t pack as much functionality.