title: How to retrieve null lookup entries on mongodb?
body: <p>I have this query that provides me the join I want to:</p>
  
  <pre><code>db.summoners.aggregate([
      { "$match": { "nick":"Luispfj" } },
      { "$unwind": "$matches" },
      {
          "$lookup": {
              "from":"matches",
              "localField":"matches.gameId",
              "foreignField":"gameId",
              "as":"fullMatches"
          }
      },
      { "$unwind": "$fullMatches" },
      {
          "$group": {
              "_id": null,
              "matches": { "$push":"$fullMatches" }
          }
      }
  ])
  </code></pre>
  
  <p>But when I run the unwind function the null entries are gone. How do I retrieve them (with their respective "gameId"s, if possible?</p>
  
  <p>Also, is there a way to retrieve only the matches array, instead of it being a subproperty of the "null-id-object" it creates?</p>