Count number of fields that match in an Array of Object in mongodb

Question

What I have

I have a DB in MongoDB like this:

{
    "_id": {
        "$oid": "60ba531acbfed3545c51a49e"
    },
    "email": "[email protected]",
    "Formats": [{
        "format": "AST-QC",
    }],
    "Series": [{
        "seq": "AST-QC - 1",
     },
     {
        "seq": "AST-QC - 2",
     },
     {
        "seq": "AST-QD - 1",
     }]
}

I am successfully getting the data from the Formats array using this query:

const pipeline =  [
    { $match: { "email": email } },
    { $unwind: "$Formats" },    
]
const res = await colc.aggregate(pipeline)

What I want

Along with the data in the Formats array, I need the count of every format that is used by seq in Series array.

I am certain that it can be done using $addFields, Something like this.

const pipeline =  [
    { $match: { "email": email } },
    { $unwind: "$Formats" },
    { $addFields: {"Formats.count": 0} }
]
const res = await colc.aggregate(pipeline)

But I am not sure as to how.

I don't want to call another query using .count()

what count you need? AST-QC - 1 concated number or matching string? — turivishal
– turivishal, Commented Jun 14, 2021 at 15:32
just the matching string from format, i.e. AST-QC not the concated number. — shaswat.dharaiya
– shaswat.dharaiya, Commented Jun 14, 2021 at 15:34

turivishal · Accepted Answer · 2021-06-14 15:39:14Z

1

$filter to iterate loop of Series array
$regexMatch to search format in seb
$size to get total elements in filtered result

const pipeline = [
  { $match: { email: email } },
  { $unwind: "$Formats" },
  {
    $addFields: {
      "Formats.count": {
        $size: {
          $filter: {
            input: "$Series",
            cond: {
              $regexMatch: {
                input: "$$this.seq",
                regex: "$Formats.format"
              }
            }
          }
        }
      }
    }
  }
]

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answered Jun 14, 2021 at 15:39

turivishal

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