[WIP]: Allow cast schemaless embeds

lib/ecto/changeset/relation.ex

@@ -103,6 +103,14 @@ defmodule Ecto.Changeset.Relation do
     end
   end
 
+  def cast(%{related: nil} = relation, owner, params, current, on_cast) do
+    fun = &do_cast(relation, owner, &1, &2, &3, &4, on_cast)
+
+    with :error <- cast_or_change(relation, params, current, fn _ -> [] end, fn _ -> [] end, fun) do
+      {:error, {"is invalid", [type: expected_type(relation)]}}
+    end
+  end
+
   def cast(%{related: mod} = relation, owner, params, current, on_cast) do
     pks = mod.__schema__(:primary_key)
     fun = &do_cast(relation, owner, &1, &2, &3, &4, on_cast)
@@ -125,6 +133,14 @@ defmodule Ecto.Changeset.Relation do
     do_cast(meta, owner, params, struct, allowed_actions, idx, on_cast)
   end
 
+  defp do_cast(%{related: nil} = relation, _owner, params, nil = _struct, allowed_actions, idx, on_cast) do
+    {:ok,
+      relation
+      |> apply_on_cast(on_cast, %{}, params, idx)
+      |> put_new_action(:insert)
+      |> check_action!(allowed_actions)}
+  end
+
   defp do_cast(relation, owner, params, nil = _struct, allowed_actions, idx, on_cast) do
     {:ok,
       relation

test/ecto/changeset/schemaless_test.exs

@@ -0,0 +1,44 @@
+defmodule Ecto.Changeset.SchemalessTest do
+  use ExUnit.Case, async: true
+
+  defmodule Author do
+    import Ecto.Changeset
+
+    @types %{
+      name: :string,
+      age: :integer,
+      book: {:embed, Ecto.Embedded.init(cardinality: :one, field: :book)}
+    }
+
+    @keys Map.keys(@types) -- [:book]
+
+    @book_types %{
+      id: :integer,
+      name: :string
+    }
+
+    @book_keys Map.keys(@book_types)
+
+    def build(entity, attrs) do
+      entity
+      |> changeset(attrs)
+      |> apply_action(:validate)
+    end
+
+    def changeset(entity, attrs) do
+      {entity, @types}
+      |> cast(attrs, @keys)
+      |> cast_embed(:book, with: &book_changeset/2)
+    end
+
+    def book_changeset(entity, attrs) do
+      {entity, @book_types}
+      |> cast(attrs, @book_keys)
+    end
+  end
+
+  test "successfully casts schemaless embed" do
+    {:ok, data} = Author.build(%{}, %{name: "Jane", book: %{id: 1, name: "title 1"}})
+    assert %{name: "Jane", book: %{id: 1, name: "title 1"}} = data
+  end
+end