How do I update a relationship in Ecto?

I have a subscription like this.

defmodule Rebirth.Subscription do
  use Rebirth.Web, :model

  schema "subscriptions" do
    ...
    belongs_to :user, Rebirth.User
    ...
  end

  ...

  def update_user(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
    |> cast_assoc(:user, required: false)    
  end
end

I want to associate a user to the subscription

So I tried

Rebirth.Subscription.update_user(subscription, %{user_id: 1})

or

Rebirth.Subscription.update_user(subscription, %{user: user})

When I run it I get the following error:

** (ArgumentError) unknown assoc `user` in `cast_assoc`

How do I update the user_id?

Thanks!