mongodb - MongoError: Unrecognized pipeline stage name: '$addFields'

Question

MongoError: Unrecognized pipeline stage name: '$addFields'. "mongoose": "^4.5.8" My sourcecode:

                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);
                    }
                )

Post model:

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: ''}
}

Answer 1

$addFields is introduced in Mongo 3.4 version. As you have commented that you are using mongo 3.2.9, the mentioned query won't work.

If you cannot update the mongo version for some reason, then you have to use following approach where you need to iterate over each document and set the new field

Post.find({}).forEach(function(post){
  post.findOneAndUpdate({_id: post._id}, 
      {$set: {userName: post.author.firstName + " " + post.author.lastName }})
});