Project nested document using regex

Question

Disclaimer: Totally new in MongoDB.

I have a document structured something like this:

{
  "hd": [
    {
      "rd": [
        {
          "rm": [
            {
              "code": "a1asd"
            },
            {
              "code": "a24aer"
            }
          ]
        },
        {
          "rm": [
            {
              "code": "b16hwe7"
            },
            {
              "code": "b2z41s"
            }
          ]
        }
      ]
    }
  ]
}

And I want return the subdocument that only contains, for example, "a1". How will I approach this problem?

The solutions I tried so far looks something like this

db.foo.aggregate([
    {
        $match: {
            "hd.rd.rm.code": /a1/
        }
    },
    {
        $project: {
            "hd.rd": {
                $filter: {
                    input: "$hd.rd",
                    as: "roomData",
                    cond: {
                        $eq: [
                            {
                                $substr: [ "$$roomData.rm.code", 0, 10]
                            },
                            "a1"
                        ]
                    }
                }
            }
        }
    }
])

but the query above doesn't work. I found out $regexMatch doesn't work with $cond, so I'm quite loss at the moment.

My expected results will look something like this:

{
  "hd": {
    "rd": {
      "rm": {
        "code": "a1asd"
      }
    }
  }
}

Mohammad Faisal · Accepted Answer · 2020-03-13 10:24:41Z

1

This query will give you the desired result,

db.collection.aggregate([
 {
    "$unwind": "$hd"
  },
  {
    "$unwind": "$hd.rd"
  },
  {
    "$unwind": "$hd.rd.rm"
  },
  {
    "$match": {
      "hd.rd.rm.code": /a1/
    }
  }
])

ScreenShot of Mongo Instance

edited Mar 13, 2020 at 10:24

answered Mar 12, 2020 at 13:16

Mohammad Faisal

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5 Comments

Mohammad Faisal Over a year ago

what are you getting? I have tested the query and it gives me the correct result.

cankentcode Over a year ago

I tried it on my mongo instance, and it doesn't return any result even tho the nested document exist.

Mohammad Faisal Over a year ago

Are you replace the collection with your collection name and using the right database as default database is test in mongo instance.

Mohammad Faisal Over a year ago

@cankentcode I have added the screenshot of the query with result. Here i added your data in a collection b

cankentcode Over a year ago

confirmed it's working. there's an error on my part. thank you

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