Fix casting embedded schema with no fields and type on parent

lib/polymorphic_embed.ex

@@ -185,9 +185,6 @@ defmodule PolymorphicEmbed do
       {:ok, nil} when not required ->
         Ecto.Changeset.put_change(changeset, field, nil)
 
-      {:ok, map} when map == %{} and not array? ->
-        changeset
-
       {:ok, params_for_field} when array? ->
         create_sort_default = fn -> sort_create(Enum.into(cast_opts, %{}), field_opts) end
         params_for_field = apply_sort_drop(params_for_field, sort, drop, create_sort_default)
@@ -365,10 +362,15 @@ defmodule PolymorphicEmbed do
         is_nil(type_from_map) ->
           module = get_polymorphic_module_for_type(type_from_parent_field, types_metadata)
 
-          if is_nil(data_for_field) or data_for_field.__struct__ != module do
-            {:insert, struct(module)}
-          else
-            {:update, data_for_field}
+          cond do
+            is_nil(module) ->
+              :type_not_found
+
+            is_nil(data_for_field) or data_for_field.__struct__ != module ->
+              {:insert, struct(module)}
+
+            true ->
+              {:update, data_for_field}
           end
 
         to_string(type_from_parent_field) != to_string(type_from_map) ->

test/polymorphic_embed_test.exs

@@ -1133,6 +1133,37 @@ defmodule PolymorphicEmbedTest do
                  end
   end
 
+  test "cast embed with a schema that has no fields" do
+    broadcast_reminder_attrs = %{
+      date: ~U[2020-05-28 02:57:19Z],
+      text: "This is a Broadcast reminder polymorphic",
+      channel: %{
+        my_type_field: "broadcast"
+      }
+    }
+
+    changeset =
+      PolymorphicEmbed.Reminder.changeset(%PolymorphicEmbed.Reminder{}, broadcast_reminder_attrs)
+
+    assert changeset.valid?
+    assert changeset.changes.channel == %PolymorphicEmbed.Channel.Broadcast{}
+  end
+
+  test "cast embed with a schema that has not fields and type is in parent" do
+    broadcast_reminder_attrs = %{
+      date: ~U[2020-05-28 02:57:19Z],
+      text: "This is a Broadcast reminder polymorphic",
+      type: "broadcast",
+      channel4: %{}
+    }
+
+    changeset =
+      PolymorphicEmbed.Reminder.changeset(%PolymorphicEmbed.Reminder{}, broadcast_reminder_attrs)
+
+    assert changeset.valid?
+    assert changeset.changes.channel4 == %PolymorphicEmbed.Channel.Broadcast{}
+  end
+
   test "cast embed after change/2 call should succeed" do
     for generator <- @generators do
       reminder_module = get_module(Reminder, generator)
@@ -3452,7 +3483,7 @@ defmodule PolymorphicEmbedTest do
   describe "types/2" do
     test "returns the types for a polymoprhic embed field" do
       assert PolymorphicEmbed.types(PolymorphicEmbed.Reminder, :channel) ==
-               [:sms, :email]
+               [:sms, :broadcast, :email]
     end
   end
 

test/support/models/polymorphic/channel/broadcast.ex

@@ -0,0 +1,14 @@
+defmodule PolymorphicEmbed.Channel.Broadcast do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @primary_key false
+
+  embedded_schema do
+    # A schema with no fields
+  end
+
+  def changeset(broadcast, params) do
+    cast(broadcast, params, ~w()a)
+  end
+end

test/support/models/polymorphic/reminder.ex

@@ -15,6 +15,7 @@ defmodule PolymorphicEmbed.Reminder do
     polymorphic_embeds_one(:channel,
       types: [
         sms: PolymorphicEmbed.Channel.SMS,
+        broadcast: PolymorphicEmbed.Channel.Broadcast,
         email: [
           module: PolymorphicEmbed.Channel.Email,
           identify_by_fields: [:address, :confirmed]
@@ -45,6 +46,7 @@ defmodule PolymorphicEmbed.Reminder do
     polymorphic_embeds_one(:channel4,
       types: [
         sms: PolymorphicEmbed.Channel.SMS,
+        broadcast: PolymorphicEmbed.Channel.Broadcast,
         email: PolymorphicEmbed.Channel.Email
       ],
       on_replace: :update,