title: MongoDB / Mongoose - aggregate with $lookup (Left Join) not working
body: <p>The DB has 9 users (collection 1).  Each user can have 0 or 1 rsvps (collection 2).  There are only 2 rsvps in the rsvps collection.</p>
  
  <p>The following code returns 9 records.  Even for the 2 users with rsvps, rsvp is an empty array [ ].</p>
  
  <p>What am I doing wrong?</p>
  
  <pre><code>    //  Gets all Users' attendance information for the Home view.
      return new Promise( (resolve, reject) =&gt; {
  
          // Get all attendance database records using mongoose.
          User.aggregate([{
              $lookup: {
                  from: "rsvps", // collection name in db
                  localField: "_id",
                  foreignField: "userId",
                  as: "rsvp"
              }
          }
          ]).exec( (err, userList) =&gt; {
              if (err) throw err;
  
              console.log(userList);
          });
  </code></pre>