title: Disable mongoid belongs_to association validation
body: <p>A have two mongoid models with a simple has_many/belongs_to relationship between them:</p>
  
  <pre><code>class Lot
    include Mongoid::Document
    has_many :journal_items
  end
  
  class JournalItem
    include Mongoid::Document
    belongs_to :lot
  end
  </code></pre>
  
  <p>The issue is that I can't create a <code>JournalItem</code> without a <code>Lot</code> as it seems that mongoid adds a non-null validation on the <code>JournalItem.lot_id</code> by default.</p>
  
  <pre><code>JournalItem.create!
  # raises validation error "Lot can't be blank"
  </code></pre>
  
  <p>How can I disable it?</p>
  
  <p>I use the mongoid master with Rails 5.</p>