title: mongodb rename keys inside an object during aggregation
body: <p>I have a mongodb aggregation query</p>
  
  <pre><code>return this.collection.aggregate([
      { $match: { _id: ObjectID(locationId) } },
      {
          $lookup:{
              from: "buildings",
              localField: "userId",
              foreignField: "ownerId",
              as: "areas"
          }
      },
      {
          $project: {     
              "location_id": "$_id",
              "locationName": "$locationName", 
              "areas": "$areas"
          }
      }
  ]).toArray();
  </code></pre>
  
  <p>where the <code>areas</code> field is an array of objects</p>
  
  <pre><code>[{
    key: value
  }]
  </code></pre>
  
  <p>Is it possible to rename this key during the aggregation process?</p>