如何用ecto实现双向外键

defmodule MyApp.Repo.Migrations.CreateUsers do
  use Ecto.Migration

  def change do
    create table(:users) do
      add :name, :string
      timestamps()
    end
  end
end

defmodule MyApp.Repo.Migrations.CreatePosts do
  use Ecto.Migration

  def change do
    create table(:posts) do
      add :title, :string
      add :user_id, references(:users, on_delete: :nothing)
      timestamps()
    end
  end
end

defmodule MyApp.User do
  use Ecto.Schema

  schema "users" do
    field :name, :string
    has_many :posts, MyApp.Post
    timestamps()
  end
end

defmodule MyApp.Post do
  use Ecto.Schema

  schema "posts" do
    field :title, :string
    belongs_to :user, MyApp.User
    timestamps()
  end
end

def create_post(attrs \\ %{}) do
  %Post{}
  |> Post.changeset(attrs)
  |> Repo.insert()
  |> update_user_posts(attrs[:user_id])
end

defp update_user_posts({:ok, %Post{user_id: user_id}}) do
  user = Repo.get(User, user_id)
  user
  |> Ecto.Changeset.change()
  |> Ecto.Changeset.put_assoc(:posts, [post])
  |> Repo.update()
end