Phoenix LiveView form does not show errors on polymorphic_embeds_many fields

Hi and thanks for the great work !

I'm trying to create a form for a schema which has polymorphic_embeds_many field using polymorphic_embed_inputs_for but the validation errors are not shown on the inner forms when I use the base input core component.

Below is a minimum example using the schemas defined in the documentation (MyApp.Channel.Email and MyApp.Channel.SMS are omitted for brevity) and the code for my LiveView.

Inputing invalid values in the inner fields does not show any error while it does for the outer field. Am I doing something wrong with the way I build my form ?

After looking around a bit, I see that the errors are properly set on the inner fields but they are not displayed and this is related to the use of Phoenix.Component.used_input?/1 in the input core component. I was able to make it work by modifying a few lines of to_form in PolymorphicEmbed.HTML.Helpers but I'd like to know if I am properly constructing my form before going there.

defmodule MyApp.Reminder do
  use Ecto.Schema
  import Ecto.Changeset
  import PolymorphicEmbed

  schema "reminders" do
    field :date, :utc_datetime
    field :text, :string

    polymorphic_embeds_many :channels,
      types: [
        sms: MyApp.Channel.SMS,
        email: MyApp.Channel.Email
      ],
      on_type_not_found: :raise,
      on_replace: :delete
  end

  def changeset(struct, values) do
    struct
    |> cast(values, [:date, :text])
    |> cast_polymorphic_embed(:channels, required: true)
    |> validate_required(:date)
  end
end

defmodule PolembedWeb.ReminderLive.Index do
  use PolembedWeb, :live_view

  import PolymorphicEmbed.HTML.Helpers
  alias MyApp.Reminder

  @impl true
  def mount(_params, _session, socket) do
    form =
      Reminder.changeset(%Reminder{}, %{
        "channels" => [
          %{"__type__" => "sms", "number" => "32"},
          %{"__type__" => "email"}
        ]
      })
      |> to_form()

    {:ok, assign(socket, form: form)}
  end

  @impl true
  def handle_event("validate", %{"reminder" => reminder_params}, socket) do
    form = Reminder.changeset(%Reminder{}, reminder_params) |> to_form(action: :validate)
    {:noreply, assign(socket, form: form)}
  end

  @impl true
  def handle_event("save", _params, socket) do
    {:noreply, socket}
  end

  @impl true
  def render(assigns) do
    ~H"""
    <.form for={@form} id="reminders-form" phx-change="validate" phx-submit="save">
      <.input field={@form[:date]} type="date" label="Name" />
      <.polymorphic_embed_inputs_for :let={channel} field={@form[:channels]}>
        <%= case source_module(channel) do %>
          <% MyApp.Channel.SMS -> %>
            <.input field={channel[:number]} type="number" label="Number" />
          <% MyApp.Channel.Email -> %>
            <.input field={channel[:address]} type="text" label="Address" />
        <% end %>
      </.polymorphic_embed_inputs_for>
    </.form>
    """
  end
end

@anjou-low

@anjou-low Hey, i can confirm the issue with Phoenix.Component.used_input?/1.
Could you elaborate what changes you did to to_form to make it work?

bump

The issue seems to come from the fact that Phoenix.Component.used_input?/1 looks at the field's params to infer if it has been used but they are not always set correctly here:

polymorphic_embed/lib/polymorphic_embed/html/helpers.ex

Line 74 in 18a506d

params = Enum.at(params, i) || %{}

If I construct the form like this

form = 
  Reminder.changeset(%Reminder{}, %{
    "channels" => [
      %{"__type__" => "sms", "number" => "32"},
      %{"__type__" => "email"}
    ]
  })
  |> to_form(action: :validate)

then

[
  %{"__type__" => "sms", "number" => "32"}, 
  %{"__type__" => "email"}
] = Map.get(form.source.params, "channels")

but as soon as the params are sent by the client we get something like

%{"inputs" => %{
  "0" => %{"__type__" => "sms", "_persistent_id" => "0", "number" => "3"},
  "1" => %{"__type__" => "email", "_persistent_id" => "1", "_unused_address" => "", ...}
}, ...
}

and this format is not handled by PolymorphicEmbed.HTML.Helpers.to_form/4.

One quick fix could be to not List.wrap the changeset's params and do something like that

params =
cond do
  is_list(params) -> Enum.at(params, i) || %{}
  is_map(params) -> Map.get(params, index_string) || %{}
end

but it feels hacky.

I think this is related to #74 because the params are properly handled by cast_polymorphic_embed/2

params = %{"channels" => %{"0" => %{"__type__" => "sms"}}}

changeset = 
  %Reminder{}
  |> Ecto.Changeset.cast(params, [])
  |> PolymorphicEmbed.cast_polymorphic_embed(:channels)

%{"__type__" => "sms"} = Map.get(changeset.changes, :inputs) |> List.first() |> Map.get(:params)

but we can't always rely on this since the changeset is transformed to a struct if it valid.

Hey,
I implemented something like you suggested in #132
Sadly this will introduce other problems:
when adding inputs using the sort_param it will populate prior params if you prepend new inputs.

If I find the time I will try to dig a bit deeper

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Phoenix LiveView form does not show errors on `polymorphic_embeds_many` fields #113

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Phoenix LiveView form does not show errors on polymorphic_embeds_many fields #113

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nduitz commented on Dec 5, 2024

JacobAlexander commented on Jun 23, 2025

anjou-low commented on Jul 28, 2025

nduitz commented on Aug 20, 2025

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Phoenix LiveView form does not show errors on `polymorphic_embeds_many` fields #113