title: How can I update/insert Object into inner List in Mongodb?
body: <pre><code>Blog {
     id:"001"
     title:"This is a test blog",
     content:"...."
     comments:[{title:"comment1",content:".."},{title:"comment2",content:"..."}]    
  }
  </code></pre>
  
  <p>comments is a inner list in blog.</p>
  
  <p>But how can I retrieve only comment1?
  and How can I insert/update a new comment into the blog?if I get a full blog and insert/update the content into comments List,then save the full blog,how to solve concurrent isuue?</p>
  
  <p>Thanks.</p>