user.ex

defmodule User do
  use GenServer

  def start_link(user_id) do
    GenServer.start_link(__MODULE__, [user_id], name: user_id)
  end

  def init(user_id) do
    User.Entity.start_link(user_id)
  end

  # ...
  
  # Callbacks

  def handle_call(%ActivateUser{}, _from, user) do
    user
    |> User.activate
  end
  
  # ...
end

user_entity.ex

defmodule User.Entity do
  use Ecto.Schema

  schema "users" do
    field :email, :string
    field :active, :boolean, default: false
  end

  @doc """
  Activate the user.
  """
  def activate(pid) do
    Agent.get_and_update(pid, fn user ->
      event = %UserActivated{user_id: user.id}
      state = User.apply(user, event)
      {event, state}
    end)
  end

  def apply(%User{} = user, %UserActivated{}) do
    user
    |> changeset(%{active: true}, [:active])
    |> apply_changes
  end
end