Hi.
I am creating a simple user CRUD with Phoenix 1.4 (App is called simply Backend).
Each user must have an email and a password. The email MUST be unique.
I expressed that in the migration and the schema:

• priv/repo/migrations/20190530142317_create_users.exs

defmodule Backend.Repo.Migrations.CreateUsers do
  use Ecto.Migration

  def change do
    create table(:users) do
      add :email, :string, null: false, unique: true
      add :password, :string, null: false
      add :is_active, :boolean, default: false, null: false

      timestamps()
    end

    create unique_index(:users, [:email])
  end
end

• lib/backend/auth/user.ex

defmodule Backend.Auth.User do
  use Ecto.Schema
  import Ecto.Changeset

  schema "users" do
    field :email, :string, null: false, unique: true
    field :password, :string, null: false
    field :is_active, :boolean, default: true

    timestamps()
  end

  @doc false
  def changeset(user, attrs) do
    user
    |> cast(attrs, [:email, :password, :is_active])
    |> validate_required([:email])
    |> unique_constraint(:email, name: :users_email_index)
  end
end

My problem is that when the validation fails, all I get is:

{
    "errors": {
        "detail": "Internal Server Error"
    }
}

I worked around one error (unique email address) as follows:

• lib/backend_web/controllers/user_controller.ex

defmodule BackendWeb.UserController do
  use BackendWeb, :controller

  alias Backend.Auth
  alias Backend.Auth.User

  action_fallback BackendWeb.FallbackController

  def index(conn, _params) do
    users = Auth.list_users()
    render(conn, "index.json", users: users)
  end

  def create(conn, %{"user" => user_params}) do
    case Auth.create_user(user_params) do
    {:ok, %User{} = user} ->
      conn
      |> put_status(:created)
      |> put_resp_header("location", Routes.user_path(conn, :show, user))
      |> render("show.json", user: user)
    {:error, %Ecto.Changeset{} = changeset} ->
      case changeset.errors do
        [{:email, _}] ->
          conn
          |> send_resp(409, "User already exists!")
      end
    end
  end

  def show(conn, %{"id" => id}) do
    user = Auth.get_user!(id)
    render(conn, "show.json", user: user)
  end

  def update(conn, %{"id" => id, "user" => user_params}) do
    user = Auth.get_user!(id)

    with {:ok, %User{} = user} <- Auth.update_user(user, user_params) do
      render(conn, "show.json", user: user)
    end
  end

  def delete(conn, %{"id" => id}) do
    user = Auth.get_user!(id)

    with {:ok, %User{}} <- Auth.delete_user(user) do
      send_resp(conn, :no_content, "")
    end
  end
end

However, I would need to write a lot of functions and cases to return a meaningful message and I am sure the Phoenix team already solved it in a smart way.

For reference, I add following files:

• lib/backend_web/controllers/fallback_controller.ex

defmodule BackendWeb.FallbackController do
  @moduledoc """
  Translates controller action results into valid `Plug.Conn` responses.

  See `Phoenix.Controller.action_fallback/1` for more details.
  """
  use BackendWeb, :controller

  def call(conn, {:error, :not_found}) do
    conn
    |> put_status(:not_found)
    |> put_view(BackendWeb.ErrorView)
    |> render(:"404")
  end
end

• lib/backend_web/views/error_view.ex

defmodule BackendWeb.ErrorView do
  use BackendWeb, :view

  # If you want to customize a particular status code
  # for a certain format, you may uncomment below.
  # def render("500.json", _assigns) do
  #   %{errors: %{detail: "Internal Server Error"}}
  # end

  # By default, Phoenix returns the status message from
  # the template name. For example, "404.json" becomes
  # "Not Found".
  def template_not_found(template, _assigns) do
    %{errors: %{detail: Phoenix.Controller.status_message_from_template(template)}}
  end
end

Thank you!

PS. I already love Phoenix! Keep up the great work, Core Team!