title: removing an object from a mongodb document
body: <p>I have a document in <code>Mongodb</code> collection, where I want to <code>remove an object</code>, using <code>title</code> key. </p>
  
  <p>I tried using <code>$unset</code>, but it only <code>removes the title key</code> not the object to which it belongs.</p>
  
  <pre><code>{
      "_id" : ObjectId("576b63d49d20504c1360f688"),
      "books" : [
          {
              "art_id" : ObjectId("574e68e5ac9fbac82489b689"),
              "title" :"abc",
              "price" : 40
          },
          {
              "art_id" : ObjectId("575f9badada0500d192c53f4"),
              "title" : "xyz",
              "price" : 20
          },
          {
              "art_id" : ObjectId("57458224d86b3d1561150f17"),
              "title" : "def",,
              "price" : 30
          }
      ],
      "user_id" : "575570c315e27d13167dfc0d"
  }
  </code></pre>