title: Mongodb $lookup Not working with _id
body: <p>wend try with this query, return the lookup is empty</p>
  
  <pre><code>db.getCollection('tests').aggregate([
      {$match: {typet:'Req'}},
      {$project: {incharge:1}},
      {$lookup:{
              from: "users",
              localField: "incharge", //this is the _id user from tests
              foreignField: "_id", //this is the _id from users
              as: "user"
      }}
  ])
  </code></pre>
  
  <p>return json</p>
  
  <pre><code>  [
      {
          "_id": "57565d2e45bd27b012fc4db9",
          "incharge": "549e0bb67371ecc804ad23ef",
          "user": []
      },
      {
          "_id": "57565d2045bd27b012fc4cbb",
          "incharge": "549e0bb67371ecc804ad21ef",
          "user": []
      },
      {
          "_id": "57565d2245bd27b012fc4cc7",
          "incharge": "549e0bb67371ecc804ad24ef",
          "user": []
      }
  ]
  </code></pre>
  
  <p>i try with this post but nothing happend 
  <a href="https://stackoverflow.com/questions/27570698/mongodb-aggregation-project-string-to-objectid">MongoDB aggregation project string to ObjectId</a>
  and with this 
  <a href="https://stackoverflow.com/questions/35518571/mongodb-lookup-with-id-as-a-foreignfield-in-php">MongoDB $lookup with _id as a foreignField in PHP</a></p>
  
  <p><strong>UPDATE</strong> </p>
  
  <p>this is the Document "users"</p>
  
  <pre><code>    {
          "_id" : ObjectId("549e0bb67371ecc804ad24ef"),
          "displayname" : "Jhon S."
      },
      {
          "_id" : ObjectId("549e0bb67371ecc804ad21ef"),
          "displayname" : "George F."
      },
      {
          "_id" : ObjectId("549e0bb67371ecc804ad23ef"),
          "displayname" : "Franc D."
      }
  </code></pre>