How to remove _id from aggregate $match & $group query

One thing before we go on - you are using Promise.all and async await - that is not a great idea.

await is going to wait for the promise to resolve on each call so you are turning what should be a parallel operation (handled by Promise.All) into a synchronous one.

Other than that I am trying to reconcile your first listing with the others.

What is it you are wanting to do - it is not clear.

Hi Julian, thanks for your response. I am glad you picked up the first issue because I want to do it right. Could you please tell me the best way to do it?

What I am trying to do is query the database and get a list of products in one query and then a list of subcategories as well which I have split into 2 queries. The results are based on the category name in the slug ie: clothing.

The list of subcategories is going to end up being a filter where a user clicks on a subcategory name and it filters products shown by that subcategory.

Just remove the await from the queries - and pass the promises to Promise.All.

As for the query - I am still not clear - your first snippet showed this

"subcats": [
	  {
		"_id": {
		  "subcategory": {
			"_id": "5d2b42c47b454712f4db7c37",
			"name": "shirts"
		  }
		}
	  }
	]

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Which does not seem to match up with the values that are being returned by your query?

If I remove await then I probably have to remove async as well and end up with this:

exports.getCatSlug = (req, res) => {
  const catslug = req.params.catslug;

  const products = Product.find({ "category.catSlug": catslug }).select({
    name: 1,
    description: 1,
    price: 1,
    category: 1
  });

  const subcats = Product.aggregate([
    { $match: { "category.catSlug": catslug } },
    { $group: { _id: { subcategory: "$subcategory.name" } } }
  ]);

  Promise.all([products, subcats]);
  res.status(200).json({
    products,
    subcats
  });

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But then I get an error
TypeError Converting circular structure to JSON
    --> starting at object with constructor 'Server'
    |     property 's' -> object with constructor 'Object'
    |     property 'coreTopology' -> object with constructor 'Server'
    |     ...
    |     property 's' -> object with constructor

My problem with the returned query is the unnecessary "_id". I don't want it in there. I highlighted it below.

"subcats": [
        {
            "_id": {
              "subcategory": {
                  "_id": "5d2b42c47b454712f4db7c37",
                  "name": "shirts"
              }
            }
        }
      ]

So, this is where I am now. I kept async/await and just didn't use 'Promise.all'. Is that okay?

exports.getCatSlug = async (req, res) => {
  const catslug = req.params.catslug;

  const products = await Product.find({ "category.catSlug": catslug }).select({
    name: 1,
    description: 1,
    price: 1,
    category: 1
  });

  const subcats = await Product.aggregate([
    { $match: { "category.catSlug": catslug } },
    {
      $group: {
        _id: "$subcategory.name"
      }
    }
  ]);
  res.status(200).json({
    products,
    subcats
  });

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I am happy with the products array I get back but the subcats array now looks like this:

    "subcats": [
        {
            "_id": "shirts"
        },
        {
            "_id": "shoes"
        },
        {
            "_id": "ties"
        }
    ]

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This is better because for now I just wanted the names which I am getting, but $group is giving me _id but I would rather it said "name". Is the only way around this to create a new array?

Thanks Julian, I have been using async await so much that I forgot that I need to use then and catch blocks  when not using async await! Besides the map part, your code gives me the same result as to what I had changed it to (not sure if you saw that post of mine but perhaps one of your comments was related to that code? So, I just wanted to find out if you were saying this was also bad practice or just the wrong way I had done the Promise.all which totally makes sense now.

exports.getCatSlug = async (req, res) => {
  const catslug = req.params.catslug;

  const products = await Product.find({ "category.catSlug": catslug }).select({
    name: 1,
    description: 1,
    price: 1,
    category: 1
  });

  const subcats = await Product.aggregate([
    { $match: { "category.catSlug": catslug } },
    {
      $group: {
        _id: "$subcategory.name"
      }
    }
  ]);
  res.status(200).json({
    products,
    subcats
  });

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The map part is what reduced the subcategory return from

"subcats": [
        {
            "_id": {
              "subcategory": {
                  "_id": "5d2b42c47b454712f4db7c37",
                  "name": "shirts"
              }
            }
        }
      ] 

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To

"subcats": [
  {
        "_id": "5d2b42c47b454712f4db7c37",
        "name": "shirts"
  }
]

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Besides the map part, your code gives me the same result as to what I had changed it to (not sure if you saw that post of mine but perhaps one of your comments was related to that code?

Your post was still using async / await. Mine used a Promise.all().then() approach with a map to return an altered subcats array.

I see you changed your query structure - which I assume is now returning the correct results.
I am not familiar with MongoDB query syntax so was not sure if your changed query was returning the data in the form that you wanted.

In event that it was not - the map code was to demonstrate how to modify an array to a different structure.

Your post was still using async / await. Mine used a Promise.all().then() approach with a map to return an altered subcats array.

Just wanted to get some clarity on the modified code below and asking if the async/await version below is still incorrect or if it is okay as I couldn't quite make out from your initial answer if it was still wrong since I no longer used Promise.all as you said:

The await solution will work but it is not optimal and not considered good practice

exports.getCatSlug = async (req, res) => {
  const catslug = req.params.catslug;

  const products = await Product.find({ "category.catSlug": catslug }).select({
    name: 1,
    description: 1,
    price: 1,
    category: 1
  });

  const subcats = await Product.aggregate([
    { $match: { "category.catSlug": catslug } },
    {
      $group: {
        _id: "$subcategory.name"
      }
    }
  ]);
  res.status(200).json({
    products,
    subcats
  });

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If the above isn't good practice or not a good way of doing it then I will use Promise.all going forward as per your code but was just wanted to make sure I understood what you were saying properly.

Yes, with the await as you have it you have turned your async calls into synch calls - so you don't need Promise.all as the promises will resolve before continuing.

Great, thanks for all your help, it is much appreciated!

You are welcome.