title: How to use aggregrate in mongodb to $match _id
body: <p>Document:</p>
  
  <pre><code>{
      "_id" : ObjectId("560c24b853b558856ef193a3"),
      "name" : "Karl Morrison",
      "pic" : "",
      "language" : ObjectId("560c24b853b558856ef193a2"),
      "cell" : 1,
      "local" : {
          "email" : "karl.morrison@instanty.se",
          "password" : "12345"
      },
      "sessions" : [
          {
              "id" : ObjectId("560c24b853b558856ef193a5")
          }
      ]
  }
  </code></pre>
  
  <p>This works:</p>
  
  <pre><code>yield new Promise(function (resolve, reject) {
                  users.col.aggregate([
                          {
                              $match: {
                                  'name': 'Karl Morrison'
                              }
                          }
                      ],
                      function (err, res) {
                          console.log('err ' + err);
                          console.log('res ' + JSON.stringify(res)); // &lt;-- echos the object retrieved
                          if (err === null)
                              resolve(res);
                          reject(err);
                      });
              });
  </code></pre>
  
  <p>This does not work:</p>
  
  <pre><code>yield new Promise(function (resolve, reject) {
                  users.col.aggregate([
                          {
                              $match: {
                                  '_id': '560c24b853b558856ef193a3' // &lt;-- _id of the user
                              }
                          }
                      ],
                      function (err, res) {
                          console.log('err ' + err);
                          console.log('res ' + JSON.stringify(res));
                          if (err === null)
                              resolve(res);
                          reject(err);
                      });
              });
  </code></pre>
  
  <p>The <code>.col</code> access the native mongodb object (using co-monk otherwise). So I'm doing it manually. This however isn't working. I suspect I am not casting the id hexstring to an ObjectId. No matter what I try nothing works.</p>