title: Insert null into ecto belongs_to field
body: <p>I have an ecto model</p>
  
  <pre><code>defmodule App.Profile  do
  use App.Web, :model
  alias App.Repo
  
  schema "profiles" do
    belongs_to  :user,            App.User
    field       :name,            :string
    field       :last_name,       :string
    field       :second_surname,  :string
  
    timestamps
  end
  </code></pre>
  
  <p>But sometime I want to save to database and put the user to nil, Need I to add some flag to the user field? </p>
  
  <p>I have this code in my controller</p>
  
  <pre><code>changeset = Profile.changeset(%Profile{user_id: nil}, profile_params)
  </code></pre>
  
  <p>But when try to save I got this error</p>
  
  <pre><code>:erlang.byte_size({:user_id, "can't be blank"})
  </code></pre>
  
  <p>What is the best way to insert null in the database?</p>
  
  <p>I am using postgres and when insert my data manually I am able to insert null in the user_id field.</p>