title: $match operator for sub document field in MongoDb
body: <p>I am trying new pipeline query of MongoDB so i try to execute below query.</p>
  
  <pre><code>{
  aggregate: 'Posts',
  pipeline: [
      { $unwind: '$Comments'},
      { $match: {'$Comments.Owner': 'Harry' }},
      {$group: {
          '_id': '$Comments._id'
          }
       }
     ]
  }
  </code></pre>
  
  <p>And nothing match to query so empty result returns. I guess problem can be on $match command . I am using dotted notation match comment Owner but not sure it is exactly true or not. Why this query does not return Ownders who is 'Harry' . I am sure it is exist in db.</p>