Update field and $push to array in one query in MongoDB

Question

I have this code:

db.basket.update(
    {'_id': ObjectId(data['basket_id'])},
    {
        'total': round(total, 2),
        '$push': {
            products': {
                'prod_id': data['prod_id'],
                'price': price,
                'amount': data['amount']
            }
        }
    }
)

Running this query gives me an error:

uncaught exception: field names cannot start with $ [$push]

Is it possible to update field in database object and push new object into array?

Answer 1

You needed to use $set for your single value update. Otherwise this is attempting to mix the form of update with a plain object and a "update" operator. MongoDB thinks this is just a plain object update and thus tells you that "$push" is illegal for a field name:

db.basket.update(
    {'_id': ObjectId(data['basket_id'])},
    {
        '$set': { 'total': round(total, 2) },
        '$push': {
            products': {
                'prod_id': data['prod_id'],
                'price': price,
                'amount': data['amount']
            }
        }
    }
)

So using the correct operator here let's MongoDB know what you are trying to do and processes it correctly. Other update operators work in the same way. Only where all together.