Mongodb nested array update query

Question

This is a sample collection in which I want to update a user's posts. In fact, I want to operate a command similar to $push and $pop so that I can remove or add the userId from the likes array inside the posts array, this seems fairly simple but the catch is that I want to update the likes of a particular post matched by _id field of objects within posts array.

[
 {
 "_id": {
 "$oid": "61375acc1c7d0a1a6e6005f0"
 },
 "fullName": "user1",
 "email": "[email protected]",
 "password": "2ドルb10ドル$Yrs5H3mYrM8xLwWlek3K7uAs.EOLsXggj6wV7oSflPlPjo1ZkFem6",
 "avatar": "https://avatars.dicebear.com/api/human/[email protected]",
 "posts": [
 {
 "postContent": "sample 1",
 "medias": [
 "files/78560be22a25988c38ddafa0be7558f73713607a.jpeg"
 ],
 "createdAt": {
 "$date": "2021年09月07日T12:40:54.930Z"
 },
 "userId": {
 "$oid": "61375acc1c7d0a1a6e6005f0"
 },
 "authorName": "user1",
 "avatar": "https://avatars.dicebear.com/api/human/[email protected]",
 "likes": [],
 "comments": [],
 "_id": {
 "$oid": "61375dd61c7d0a1a6e6005f3"
 }
 },
 {
 "postContent": "sample 2",
 "medias": [
 "files/5af7e5086e8dfb8af6c8e7bac3f2430a78d80ac1.jpeg",
 "files/c01ed66cef55c876c8e7800aa5c820b9aeee7267.jpeg"
 ],
 "createdAt": {
 "$date": "2021年09月07日T12:42:01.142Z"
 },
 "userId": {
 "$oid": "61375acc1c7d0a1a6e6005f0"
 },
 "authorName": "user 2",
 "avatar": "https://avatars.dicebear.com/api/human/[email protected]",
 "likes": [],
 "comments": [],
 "_id": {
 "$oid": "61375e191c7d0a1a6e6005f4"
 }
 }
 ],
 "friendRequests": []
 }
]

i.e, once a query is executed the likes array of the particular post (matched by _id) has to be updated with the userId (both userId and postId will be input in this case)

eg:

document.posts.likes = [61375acc1c7d0a1a6e6005f0, 61375acc1c7d0a1aasdfasgre3]

• mohammad Naimi: you want to push posts.userId in posts.like ?
• Vishnu Mohan: No read the last part... Likes array inside posts array objects should store id of users who liked the respective posts

Answer 1

There is no straight way to do this, you can try update with aggregation pipeline starting from MongoDB 4.2,

• $map to iterate look of posts array
• $cond to check if post id match then go to update part otherwise return same object
• $cond to check if user id in likes array then go to remove part otehrwise add id in likes
• $filter to iterate loop of likes array and remove user id
• $concatArrays add user id in likes array
• $mergeObjects to merge current object with updated likes array field

let postId = { "$oid": "61375e191c7d0a1a6e6005f4" };
let userId = "61375dd61c7d0a1a6e6005f1";
db.collection.updateOne(
 { "posts._id": postId },
 [{
 "$set": {
 "posts": {
 $map: {
 input: "$posts",
 as: "p",
 in: {
 $cond: [
 { $eq: ["$$p._id", postId ] },
 {
 $mergeObjects: [
 "$$p",
 {
 "likes": {
 "$cond": [
 { $in: [userId, "$$p.likes"] },
 {
 $filter: {
 input: "$$p.likes",
 cond: { $ne: ["$$this", userId] }
 }
 },
 {
 "$concatArrays": ["$$p.likes", [userId]]
 }
 ]
 }
 }
 ]
 },
 "$$p"
 ]
 }
 }
 }
 }
 }]
)

Playground