title: How to remove has_one association in Rails 4?
body: <p>There are related models:</p>
  
  <pre><code>class ItemOwner
    include Mongoid::Document
    has_one :item
  end
  
  class Item
    include Mongoid::Document
    belongs_to :item_owner
  end
  </code></pre>
  
  <p>I tried to link Item to ItemGroup. And then I tried to link another Item.</p>
  
  <pre><code>2.5.1 :001 &gt; owner = ItemOwner.create
   =&gt; #&lt;ItemOwner _id: 5d7b80ff8329b56e1e4383f8, &gt; 
  2.5.1 :002 &gt; owner.create_item
  "item_owner_id"=&gt;BSON::ObjectId('5d7b80ff8329b56e1e4383f8')}], "ordered"=&gt;true}
   =&gt; #&lt;Item _id: 5d7b81188329b56e1e4383f9, item_owner_id: BSON::ObjectId('5d7b80ff8329b56e1e4383f8')&gt; 
  2.5.1 :003 &gt; owner.create_item
  "item_owner_id"=&gt;BSON::ObjectId('5d7b80ff8329b56e1e4383f8')}], "ordered"=&gt;true}
   =&gt; #&lt;Item _id: 5d7b81208329b56e1e4383fa, item_owner_id: BSON::ObjectId('5d7b80ff8329b56e1e4383f8')&gt; 
  2.5.1 :004 &gt; owner.reload.item
   =&gt; #&lt;Item _id: 5d7b81188329b56e1e4383f9, item_owner_id: BSON::ObjectId('5d7b80ff8329b56e1e4383f8')&gt; 
  2.5.1 :007 &gt; Item.all.pluck(:item_owner_id)
   =&gt; [BSON::ObjectId('5d7b80ff8329b56e1e4383f8'), BSON::ObjectId('5d7b80ff8329b56e1e4383f8')] 
  </code></pre>
  
  <p>Now there are two items in the database and both related to one ItemOwner.</p>
  
  <p>What is the right way to replace one item with another?</p>