Mongodb not storing string array in document - node.js

Question

If I add console.log(req.body.biddingGroup) to my PUT method then
it returns
[ [ { bidderId: '5dd5b31213372b165872bf5b' } ] ]
Therefore I know that the value is being passes properly, but if I add the line biddingGroup: req.body.biddingGroup
the bid never updates to mongodb. If I remove that one line, then the bid updates.
Not sure why it's not being passed to mongodb. I also tried in monogoose model
biddingGroup: [{
type: String
}]
but that returned the error
Cast to string failed for value "{}" at path "biddingGroup"
mongoose schema
const mongoose = require('mongoose');
const postSchema
= mongoose.Schema({
title: {type: String},
startingBid: {type: String},
bidderId: {type: String},
currentBid: {type: String},
lastBidTimeStamp: {type: Date},
increments: {type: String},
shippingCost: {type: String},
auctionType: {type: String},
buyItNow: {type: String},
snipingRules: {type: String},
auctionEndDateTime: {type: String},
biddingGroup: {bidderId: {type: String}},
currentBid: { type: String, require: true },
lastBidTimeStamp: { type: Date, required: true },
creator: { type: mongoose.Schema.Types.ObjectId, ref: "User"},
});
const Auction = mongoose.model('Listing', postSchema);
module.exports = Auction;
app.js
app.put('/api/listings/:id', (req, res) => {
console.log(req.body.biddingGroup);
Post.findByIdAndUpdate({ _id: req.params.id },
{
currentBid: req.body.currentBid,
lastBidTimeStamp: Date.now(),
bidderId: req.body.bidderId,
biddingGroup: req.body.biddingGroup,
auctionEndDateTime: req.body.auctionEndDateTime
}, function (err, docs) {
if (err) res.json(err);
else {
console.log(docs)
}
});
});
I appreciate any help!

Answer 1

From your mongoose schema you are specifying the "biddingGroup" to be an array of strings, but what you actually get (based on your console output) is an array of arrays of objects, each of those objects then should have a property called bidderId, which is a string.
To achieve a schema that matches what you get in your console.log, you can do the following:
const postSchema = new mongoose.Schema({
...
biddingGroup: [[{bidderId: String}]],
...});