In my application, users can upload multiple Contracts as files for a User from a form.

defmodule User do
  use Ecto.Schema
  import Ecto.Changeset

  @primary_key false
  embedded_schema do
    field :name, :string

    embeds_many :contracts, Contract, primary_key: false, on_replace: :delete do
      field :file_url, :string
    end
  end

  def changeset(%__MODULE__{} = struct, attrs) do
    struct
    |> cast(attrs, [:name])
    |> cast_embed(
      :contracts,
      with: &changeset_contract/2,
      sort_param: :contracts_sort,
      drop_param: :contracts_drop
    )
  end

  def changeset_contract(%__MODULE__.Contract{} = struct, attrs) do
    struct
    |> cast(attrs, [:file_url])
  end
end

I want to handle this using the Dynamic Form feature of LiveView.

defmodule MyAppWeb.Live do
  ...
  @impl true
  def render(assigns) do
    ~H"""
    <.simple_form for={@user_form} phx-change="validate" phx-submit="submit">
      <.input label="name" field={@user_form[:name]} />
      <.inputs_for :let={fc} field={@user_form[:contracts]}>
        <input type="hidden" name="user[contracts_sort][]" value={fc.index} />

        <.live_file_input upload={???} /> # <-- this is the problem

        <.label>
          <input type="checkbox" name="user[contracts_drop][]" value={fc.index} class="hidden" />
          <.icon name="hero-x-mark" class="w-6 h-6 relative top-2" /> remove
        </.label>
      </.inputs_for>

      <label class="block cursor-pointer">
        <input type="checkbox" name="user[contracts_sort][]" class="hidden" />
        <.icon name="hero-plus-circle" class="mr-1 align-text-bottom" />add more
      </label>

      <:actions>
        <.button>Submit</.button>
      </:actions>
    </.simple_form>
  end
end

In the validation event, it’s possible to dynamically add file uploads using allow_upload/3 . However, due to Contracts being dynamically created and deleted, the index keeps changing (what was originally the 3rd index becomes the 2nd index if the 1st index is deleted), making it difficult to track. I would like to use the _persistent_id, which does not change, but it’s hard to track since _persistent_id is unknown at the point when the input is added.

What is the best way to handle file uploads in a LiveView dynamic form?