my_schema.ex

defmodule MySchema do
  use Ecto.Schema

  import Ecto.Changeset

  @type t :: %__MODULE__{}

  schema "my_schemas" do
    field(:name, :string)

    embeds_one(:permissions, MembershipPermissions,
      primary_key: false,
      on_replace: :update
    ) do
      field(:can_do_stuff, :boolean, default: true)
    end

    timestamps(type: :utc_datetime)
  end

  def changeset(schema, attrs) do
    schema
    |> cast(attrs, [:name])
    |> set_default_permissions()
    |> cast_embed(:permissions, with: &permissions_changeset/2, required: true)
  end

  def set_default_permissions(changeset) do
    case get_field(changeset, :permissions) do
      nil ->
        put_embed(changeset, :permissions, %{})

      _ ->
        changeset
    end
  end

  def permissions_changeset(schema, params) do
    schema
    |> cast(params, [:can_do_stuff])
    |> validate_required([:can_do_stuff])
  end
end