how to match _id and perticular array element in mongodb without using unwind-CodePudding

user data includes :

"_id" : 4
"username" : "smith"
likedVideos :[
           "videoId" : 10
           "title" : "something"
           "speaker" : "something"
           "description" : "something"
         ]

i have a collection with userId and a array of liked videos lists.liked videos(Array) includes videoId and video details. so i need to check that is the user is already liked this video or not. so how i match userId and videoId from the collection without using unwind ?

i tried :

const data = await userModel.findOne({ _id : userId,likedVideos: { $elemMatch: { videoId : videoId } } }) but it returns all the data of that user.
const alreadyLiked = await userModel.aggregate([ { $match: { '_id' : userId, 'likedVideos.videoId' : videoId, }, }, ]);

this also not working as expected.

I need a perfect solution to match a element inside array without using unwind (My boss said that unwind is a costly operation it will effect the app performance). can you please help me to solve this.

CodePudding user response：

Both of your queries are valid. It will return all the users that match your query. You are matching a specific user and a movie. So if a user is returned, it means that the user has already liked the video.

Nevertheless $elemMatch is useful when you have multiple conditions to apply to an object. So it's better to use your second solution with

{
  "_id": userId,
  "likedVideos.videoId": videoId
}

If you want to keep only a given element in likedVideos, you can use $filter in an aggregate.

For example

db.collection.aggregate([
  {
    $match: {
      "_id": 1
    }
  },
  {
    $project: {
      list: {
        $filter: {
          input: "$likedVideos",
          as: "item",
          cond: {
            $eq: [
              "$$item.videoId",
              1
            ]
          }
        }
      }
    }
  }
])

it will only return the movies in likedVideoswith id=1

try it here

CodePudding user response：

The best way to filter elements in a subarray is by using an Aggregation with $match and $project.

Example:

[{
  $match: { 
    _id: 'userId',
    likedVideos.videoId: 'videoId'
  }
}, {
  $project: {
    'likedVideos': {
      $filter: {
          input: '$likedVideos',
          as: 'item',
          cond: 
             {$eq: ["$$item.videoId","videoId"]}
      }
    }
  }
}]