猫鼬聚合未返回预期结果

Question

I am currently learning aggregation and some related methods, after following mongoose and mongodb docs i tried it. but i am having problem.

Follow.aggregate([
            { $match: { user: mongoose.Types.ObjectId(userId) } },
            { $unwind: '$followers' },
            {
                $lookup: {
                    from: 'accounts',
                    localField: 'followers',
                    foreignField: '_id',
                    as: 'followers'
                }
            },
          { $project: { name: 1, photo: 1 } },

        ]).exec((err, followers) => {
            if (err) throw err;
            console.log(followers);
            res.send(followers);
        });

I want to get the followers of that userID and select the followers names and photo, but i am only getting the objectid of the matched document

[ { _id: 5bfe2c529419a560fb3e92eb } ]

expected output

 [ { _id: 5bfe2c529419a560fb3e92eb , name: 'john doe", photo: 'cat.jpg'} ]

Follow Model

const FollowSchema = new Schema({
    user: {
        type: Schema.Types.ObjectId,
        ref: 'Account',
        required: true,
    },
    following: [{
        type: Schema.Types.ObjectId,
        ref: 'Account',
        required: true,
    }],
    followers: [{
        type: Schema.Types.ObjectId,
        ref: 'Account',
        required: true,
    }],
});

Answer 1

When the $project stage is wrong you would get that result. Try removing it first and see the output. To me it seems you should have something like this there:

{ $project: { "followers.name": 1, "followers.photo": 1 } },