title: Update parent object when child objects meet certain condition
body: <p>I have 2 models - Order and Item:</p>
  
  <p>order.rb:</p>
  
  <pre><code>class Order &lt; ActiveRecord::Base
    has_many :items
  end
  </code></pre>
  
  <p>item.rb:</p>
  
  <pre><code>class Item &lt; ActiveRecord::Base
  belongs_to :order
  end
  </code></pre>
  
  <p>schema for item:</p>
  
  <pre><code>t.decimal  "price",       precision: 12, scale: 3
  t.string   "status"
  </code></pre>
  
  <p>schema for order:</p>
  
  <pre><code> t.string   "status"
  </code></pre>
  
  <p>Item's status will be marked as shipped when user received the items. So how can I update my order status to "complete",  in the condition that all items' status are updated to "shipped"? </p>