How to get likes count in Mongoose

Question

I have 3 tables, Users, Posts, and Likes, the schemas of them just like

Users: 
id: ObjectId; nickname: string; ...

Posts:
id: ObjectId; title: string; authorId: ObjectId(relate to Users id); ...

Likes:
id: ObjectId; forId: ObjectId(relate to Posts id); ...

I'd to get the amount of all likes amount of a specific user, How can I do it? Can anyone help me, I'm using mongoose in Node.js. (I get all posts of the user and loop them then calculate the likes amount currently, but I don't think it's a good solution)

Answer 1

You can $lookup Posts from Users in an aggregation pipeline. Then, repeat $lookup again Likes from the result. Finally, group by id of user and count the total number of likes.

db.Users.aggregate([
  {
    $match: {
      "_id": 1
    }
  },
  {
    $lookup: {
      from: "Posts",
      localField: "_id",
      foreignField: "authorId",
      as: "post"
    }
  },
  {
    $unwind: {
      path: "$post",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $lookup: {
      from: "Likes",
      localField: "post._id",
      foreignField: "forId",
      as: "like"
    }
  },
  {
    $unwind: {
      path: "$like",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $group: {
      _id: "$_id",
      likeCount: {
        $sum: 1
      }
    }
  }
])

Here is a Mongo Playground