title: MongoError: Unrecognized pipeline stage name: '$addFields'
body: <p>MongoError: Unrecognized pipeline stage name: '$addFields'.
  "mongoose": "^4.5.8"
  My sourcecode:</p>
  
  <pre><code>                Post.aggregate(
                      [{
                          $addFields: {
                              userName: { $concat: [ "$author.firstName", " ", "$author.lastName" ] }
                          }
                          //$project: { userName: { $concat: [ "$author.firstName", " ", "$author.lastName" ] } } //this is ok!
                      }],
                      function (err, result) {
                          if (err) {
                              console.log(err);
                              return;
                          }
                          console.log(result);
                      }
                  )
  </code></pre>
  
  <p>Post model:</p>
  
  <pre><code>let schema = {
  id: "post",
  properties: {
      content: {type: "string"},
      author: {
          type: "object",
          id: {type: "string"},
          avatar: {type: "string"},
          firstName: {type: "string"},
          lastName: {type: "string"},
          status: {type: "string"}
      },
      category: {
          type: "object",
          id: {type: "string"},
          name: {type: "string"}
      },
      images: {
          type: "array",
          items: {
              type: "object",
              properties: {
                  filePath: {type: "string"},
              }
          }
      },
      video: {
          type: "object",
          thumbnail: {type: "string"},
          filePath: {type: "string"}
      },
      likes: {
          type: "array",
          items: {
              type: "object",
              properties: {
                  userId: {type: "string"},
                  status: {type: "string"},
                  _id   : {type: "string", default: null}
              }
          }
      },
      shares: {
          type: "array",
          items: {
              type: "object",
              properties: {
                  userId: {type: "string"},
                  status: {type: "string"},
                  destination: {type: "string"}, //FACEBOOK|TWISTER|GOOGLE
                  _id        : {type: "string", default: null}
              }
          }
      },
      favorites: {
          type: "array",
          items: {
              type: "object",
              properties: {
                  userId: {type: "string"},
                  status: {type: "string"},
                  _id   : {type: "string", default: null}
              }
          }
      },
      comments: {
          type: "array",
          items: {
              type: "object",
              properties: {
                  commentId: {type: "string"},
                  _deleted: {type: "Date", default: ''},
                  _id     : {type: "string", default: null}
              }
          }
      },
      _created: {type: "Date", default: Date.now},
      _deleted: {type: "Date", default: ''},
      _updated: {type: "Date", default: ''}
  }
  </code></pre>