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lib/nulla_web/controllers/fallback_controller.ex

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+defmodule NullaWeb.FallbackController do
+  @moduledoc """
+  Translates controller action results into valid `Plug.Conn` responses.
+
+  See `Phoenix.Controller.action_fallback/1` for more details.
+  """
+  use NullaWeb, :controller
+
+  # This clause handles errors returned by Ecto's insert/update/delete.
+  def call(conn, {:error, %Ecto.Changeset{} = changeset}) do
+    conn
+    |> put_status(:unprocessable_entity)
+    |> put_view(json: NullaWeb.ChangesetJSON)
+    |> render(:error, changeset: changeset)
+  end
+
+  # This clause is an example of how to handle resources that cannot be found.
+  def call(conn, {:error, :not_found}) do
+    conn
+    |> put_status(:not_found)
+    |> put_view(html: NullaWeb.ErrorHTML, json: NullaWeb.ErrorJSON)
+    |> render(:"404")
+  end
+end