Mongoose - Update a document's map

Question

I have the following schema:

    ...
    friends: {
        type: Map,
        of: String
    },
    ...

I am creating a game where you can add friends, so I want to store friend's username and ID in that map. When I do

const requestantFriends = await UserData.findOne({userId: requestantData._id})

await UserData.findOneAndUpdate({userId: requestantData._id}, {friends: requestantFriends.friends.set(targetData._id, "test")}, {new: true})

nothing seems to happen. If I check the database, it still says Map(0) {} . How can I update a document's map and save it?

I've also tried

const requestantFriends = await UserData.findOne({userId: requestantData._id})

await requestantFriends.friends.set(targetData._id, "test")
await requestantFriends.save()

Answer 1

I'm assuming the 'friends' map is a field in the UserData model.

Here's the code!

await UserData.findOneAndUpdate({userId: requestantData._id}, {
  $set: {
    [`friends.${targetData._id}`]: "test",
  }
});

First, we find the document whose userId matches requestantDate._id . Then, we use the MongoDB $set operator to add a new key-value pair. We wrap the key in brackets to indicate that its value incorporates a variable, and then we use backticks to create a template literal so that we can insert the variable key name. Finally, we provide a value and close everything out!

Answer 2

MongoDB stores data using BSON , which does not have a Map type.

When Mongoose saves a document containing a map, it is stored in MongoDB as an embedded object.

If you retrieve the object using find , Mongoose will reconstitute the object as a map, and you would then need to use get and set to modify the contents.

When using findOneAndUpdate , or any of the update variants, Mongoose sends the operation to MongoDB for it to perform on the server side.

The operation that MongoDB would understand is $set , so the update should look like:

await UserData.findOneAndUpdate({userId: requestantData._id}, {$set: {"friends._id": "test"})