Get index of array when filter in mongodb

Question

I'm going to make a query to get data with index of subarray which number is even number.

Input value:

{
    "_id" : ObjectId("5b3b206c4a25da19d05f41a2"),
    "numbers" : [ 
        {
            "name" : "Foobar",
            "lines" : [ 
                {
                    "number" : 6,
                }, 
                {
                    "number" : 11,

                },
                {
                    "number" : 15,

                },
                {
                    "number" : 8,
                }

            ]
        }
    ]
}

Desired output:

{
    "_id" : ObjectId("5b3b206c4a25da19d05f41a2"),
    "numbers" : [ 
        {
            "name" : "Foobar",
            "lines" : [ 
                {
                    "index"  : 0,
                    "number" : 6,
                },
                {
                    "index"  : 3,
                    "number" : 8,
                }
            ]
        }
    ]
}

The most important is to get the index of items of "lines".

Ashh · Accepted Answer · 2018-12-12 18:59:13Z

3

You need to use $map to iterate over the first array and then $filter with the nested array.

db.collection.aggregate([
  { "$project": {
    "numbers": {
      "$map": {
        "input": "$numbers",
        "as": "nn",
        "in": {
          "name": "$$nn.name",
          "lines": {
            "$filter": {
              "input": {
                "$map": {
                  "input": "$$nn.lines",
                  "as": "ll",
                  "in": {
                    "number": "$$ll.number",
                    "index": { "$indexOfArray": ["$$nn.lines.number", "$$ll.number"] }
                  }
                }
              },
              "as": "ll",
              "cond": { "$eq": [{ "$mod": ["$$ll.number", 2] }, 0] }
            }
          }
        }
      }
    }
  }}
])

Even if you need some more aggregation trick

db.collection.aggregate([
  { "$project": {
    "numbers": {
      "$map": {
        "input": {
          "$map": {
            "input": "$numbers",
            "as": "nn",
            "in": {
              "name": "$$nn.name",
              "lines": {
                "$filter": {
                  "input": { "$zip": { "inputs": ["$$nn.lines", { "$range": [0, { "$size": "$$nn.lines" }] }] }},
                  "as": "ll",
                  "cond": {
                    "$let": {
                      "vars": { "num": { "$arrayElemAt": ["$$ll", 0] }},
                      "in": { "$eq": [{ "$mod": ["$$num.number", 2] }, 0] }
                    }
                  }
                }
              }
            }
          }
        },
        "as": "nn",
        "in": {
          "name": "$$nn.name",
          "lines": {
            "$map": {
              "input": "$$nn.lines",
              "as": "kk",
              "in": {
                "number": { "$arrayElemAt": ["$$kk.number", 0] },
                "index": { "$arrayElemAt": ["$$kk", 1] }
              }
            }
          }
        }
      }
    }
  }}
])

Finally both will give you similar Output

[
  {
    "_id": ObjectId("5b3b206c4a25da19d05f41a2"),
    "numbers": [
      {
        "name": "Foobar",
        "lines": [
          {
            "index": 0,
            "number": 6
          },
          {
            "index": 3,
            "number": 8
          }
        ]
      }
    ]
  }
]

edited Dec 12, 2018 at 18:59

answered Dec 12, 2018 at 18:11

Ashh

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7 Comments

Kirill Over a year ago

Thank you. But can I add field with 'index"? I told that most important is to get index of the items in subarray filed to result.

Ashh Over a year ago

Yes IndexOfArray can do this. Just a min

Ashh Over a year ago

Updated my answer

Kirill Over a year ago

I try it with the mongoplayground.net/p/QkMbpaXlXH0. But there's an small error. MongoError: invalid operator '$indexOfArray' Could you let me know the reason? Thank you. :)

Ashh Over a year ago

Mongodb version?

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