Disable mongoid belongs_to association validation
<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>