Count documents based on Array value and inner Array value

Question

Before I explain my use case, I'd like to state that yes, I could change this application so that it would store things in a different manner or even split it into 2 collections for that matter. But that's not my intention, and I'd rather want to know if this is at all possible within MongoDB (since I am quite new to MongoDB). I can for sure work around this problem if I'd really need to, but rather looking for a method to achieve what I want (no I am not being lazy here, I really want to know a way to do this).

Let's get to the problem then. I have a document like below:

{
    "_id" : ObjectId("XXXXXXXXXXXXXXXXXXXXX"),
    "userId" : "XXXXXXX",
    "licenses" : [
            {
                    "domain" : "domain1.com",
                    "addons" : [
                            {"slug" : "1"},
                            {"slug" : "2"}
                    ]
            },
            {
                    "domain" : "domain2.com",
                    "addons" : [
                            {"slug" : "1"},
                    ]
            }
    ]
}

My goal is to check if a specific domain has a specific addon. When I use the below query to count the documents with domain: domain2.com and addon slug: 2 the result should be: 0. However with the below query it returns 1. I know that this is because the query is executed document wide and not just the license index that matched domain2.com. So my question is, how to do a sub $and (or however you'd call it)?

db.test.countDocuments(
    {$and: [
        {"licenses.domain": "domain2.com"}, 
        {"licenses.addons.slug": "2"}, 
    ]}
)

Basically I am looking for something like this (below isn't working obviously), but below should return 0, not 1:

db.test.countDocuments(
    {$and: [
        {
            "licenses.domain": "domain2.com",
            $and: [
                { "licenses.addons.slug": "2"} 
            ]
        }
    ]}
)

I know there is $group and $filter operators, I have been trying many combinations to no avail. I am lost at this point, I feel like I am completely missing the logic of Mongo here. However I believe this must be relatively easy to accomplish with a single query (just not for me I guess).

I have been trying to find my answer on the official documentation and via stack overflow/google, but I really couldn't find any such use case.

Any help is greatly appreciated! Thanks :)

Joe · Accepted Answer · 2020-06-16 01:28:21Z

1

What you are describe is searching for a document whose array contains a single element that matches multiple criteria.

This is exactly what the $elemMatch operator does.

Try using this for the filter part:

{
   licenses: {
    $elemMatch: {
      domain: "domain2.com",
      "addons.slug": "2"
    }
  }
}

answered Jun 16, 2020 at 1:28

Joe

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2 Comments

Rens Tillmann Over a year ago

Hi, thanks for sharing this, however I just tried this and it's not working, what do you mean with "Try using this for the filter part"? The countDocuments() only takes a <query>, and <options>. Perhaps you can update your answer to reflect the complete command line? Thank you sir!

Rens Tillmann Over a year ago

Nevermind, I believe I got it working now with the code you provided. Awesome!

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