mongodb how to query records that have at least N key in giving keys?

How to find records the have at least N same keys with the specific record?

For example the specific record has k1, k2, k3, k4, k5 five keys. How to find records that has at least 3 keys than in the five keys?

For example current data is below and find docs that has at least 3 keys in k1,k2,k3,k4

doc1 {k2:1, k3:1, k4:1, k5:1, k6:1}
doc2 {k1:1, k3:1, k4:1, k5:1, k7:1}
doc3 {k1:1, k2:1, k4:1, k5:1, k8:1}
doc4 {k1:1, k2:1, k3:1, k5:1}
doc5 {k1:1, k2:1}

expected output:

doc1 {k2:1, k3:1, k4:1}
doc2 {k1:1, k3:1, k4:1}
doc3 {k1:1, k2:1, k4:1}
doc4 {k1:1, k2:1, k3:1}

Solution

You can do the followings in an aggregation pipeline:

use $objectToArray to convert the $$ROOT document into an array of k-v tuples
use $reduce to count for array entries that the field are in [k1,k2,k3,k4]
keep only the count >= 3(or any other threshold you specified)
$project to your desired form

db.collection.aggregate([
  {
    "$addFields": {
      "validCount": {
        "$objectToArray": "$$ROOT"
      }
    }
  },
  {
    "$addFields": {
      "validCount": {
        "$reduce": {
          "input": "$validCount",
          "initialValue": 0,
          "in": {
            "$cond": {
              "if": {
                "$in": [
                  "$$this.k",
                  [
                    "k1",
                    "k2",
                    "k3",
                    "k4"
                  ]
                ]
              },
              "then": {
                "$add": [
                  "$$value",
                  1
                ]
              },
              "else": "$$value"
            }
          }
        }
      }
    }
  },
  {
    "$match": {
      validCount: {
        $gte: 3
      }
    }
  },
  {
    "$project": {
      _id: 1,
      k1: 1,
      k2: 1,
      k3: 1,
      k4: 1
    }
  }
])

Here is a Mongo playground for your reference.