title: limit fields in $lookup aggregation
body: <p><strong>MongoDB DB version 3.4.18.</strong></p>
  
  <p>I want to combine data from a foreign collection, but I don't want to have it's all keys, I want to make a limit, like this:</p>
  
  <pre><code>db.users.find({}, {password:0}, (err, docs) =&gt; {});
  </code></pre>
  
  <p>And this is my aggregation query:</p>
  
  <pre><code>let query = [
    {
      // match from first collection (activities)
      $match: {
        "_id": ObjectId("5be46f266e4a182384b3ae6a")
      }
    },
    {
      // limit fields from first collection (activities)
      $project: {
        "_user": 1,
        date: 1
      }
    },
    {
      // join a foreign collection - but how to avoice the password key? for example.
      $lookup: {
        from: "users",
        localField: "_user",
        foreignField: "_id",
        as: "user"
      }
    }
  ];
  db.activities.aggregate(query, (err, docs) =&gt; {
    console.log(docs[0].user);
    return cast.ok();
  });
  </code></pre>
  
  <p>And I have a sub question.</p>
  
  <p>Why use <code>find</code> if I can use <code>aggregate</code> with a <code>$match</code>, instead of a query</p>