Unable to select specific field for MongoDB find operation

Question

I am trying to select only one field from a mongo document and print the value for it. I found this answer https://stackoverflow.com/a/25589150 which showed how we can achieve this. Below I have tried doing the same yet the entire document ends up getting printed.

const mongoHost =
  'somemongourl'
const mongodb = require('mongodb');
const { MongoClient } = mongodb;

MongoClient.connect(
  mongoHost,
  { useNewUrlParser: true },
  async (error, client) => {
    if (error) {
      return console.log('Unable to connect to database!');
    }
    const db = client.db('cartDatabase');

    const values = await db
      .collection('cart')
      .find({ customer_key: 'c_1' }, { customer_key: 1, _id: 0 })
      .toArray();

    console.log(values);
  }
);

This is the output for example I got :-

[
  {
    _id: new ObjectId("611b7d1a848f7e6daba69014"),
    customer_key: 'c_1',
    products: [ [Object] ],
    coupon: '',
    discount: 0,
    vat: 0,
    cart_total: 999.5,
    cart_subtotal: 999.5
  }
]

This is what I was expecting -

[
  {
    customer_key: 'c_1'
  }
]

The options parameter may require a top-level `projection` property. Try `{projection: {customer_key: 1, _id: 0}}` instead. — B. Fleming, Dec 23 '21 at 15:03
Thanks a lot @B.Fleming this solves my question. Should I add this as an answer and accept it? — NUMBART, Dec 23 '21 at 15:06
Also, I had tried searching a few other resources such as this - https://docs.mongodb.com/manual/tutorial/project-fields-from-query-results/ . But could not find the mention of projection key and then assigning it the options as value. Where could I have found the documentation for this? — NUMBART, Dec 23 '21 at 15:10
I'll add a full, proper answer with relevant documentation :) — B. Fleming, Dec 23 '21 at 15:19

score 1 · Accepted Answer · answered Dec 23 '21 at 15:28

The standard Node.js MongoDB driver requires a top-level projection property for the options parameter if you wish to project your documents. This would result in the second parameter of your find() call looking like this:

{ projection: { customer_key: 1, _id: 0 } }

This is indicated in the Node.js MongoDB driver API documentation, which is notably not a 1-to-1 match with the MongoDB shell API.

As of the time of this answer, you could find the collection.find() reference here. This reference shows the following method signature (again as of when this answer was written):

find(filter: Filter<WithId<TSchema>>, options?: FindOptions<Document>)

Following the FindOptions parameter takes us to this reference page, which details the various top-level options properties available for the find() method. Among these is the projection property in question.

In short, don't use the normal MongoDB documentation as a reference for your programming language's MongoDB driver API. There will often be disconnects between the two.

Unable to select specific field for MongoDB find operation

1 Answers1