Find Index of first Matching Element $gte with $indexOfArray

Question

MongoDB has $indexOfArray to let you find the element's array index, for example:

$indexOfArray: ["$article.date", ISODate("2019-03-29")]

Is it possible to use comparison operators with $indexOfArray together, like:

$indexOfArray: ["$article.date", {$gte: ISODate("2019-03-29")}]

Answer 1

Not it's not possible with $indexOfArray as that will only look for an equality match to an expression as the second argument.

Instead you can make a construct like this:

db.data.insertOne({
    "_id" : ObjectId("5ca01e301a97dd8b468b3f55"),
    "array" : [
            ISODate("2018-03-01T00:00:00Z"),
            ISODate("2018-03-02T00:00:00Z"),
            ISODate("2018-03-03T00:00:00Z")
    ]
})

db.data.aggregate([
  { "$addFields": {
    "matchedIndex": {
      "$let": {
        "vars": {
          "matched": {
            "$arrayElemAt": [
              { "$filter": {
                "input": {
                  "$zip": {
                    "inputs": [ "$array", { "$range": [ 0, { "$size": "$array" } ] }]
                  }
                },
                "cond": { "$gte": [ { "$arrayElemAt": ["$$this", 0] }, new Date("2018-03-02") ] }
              }},
              0
            ]
          }
        },
        "in": {
          "$arrayElemAt": [{ "$ifNull": [ "$$matched", [0,-1] ] },1]
        }
      }
    }
  }}
])

Which would return for the $gte of Date("2018-03-02"):

{
        "_id" : ObjectId("5ca01e301a97dd8b468b3f55"),
        "array" : [
                ISODate("2018-03-01T00:00:00Z"),
                ISODate("2018-03-02T00:00:00Z"),
                ISODate("2018-03-03T00:00:00Z")
        ],
        "matchedIndex" : 1
}

Or -1 where the condition was not met in order to be consistent with $indexOfArray.

The basic premise is using $zip in order to "pair" with the array index positions which get generated from $range and $size of the array. This can be fed to a $filter condition which will return ALL matching elements to the supplied condition. Here it is the first element of the "pair" ( being the original array content ) via $arrayElemAt matching the specified condition using $gte

{ "$gte": [ { "$arrayElemAt": ["$$this", 0] }, new Date("2018-03-02") ] }

The $filter will return either ALL elements after ( in the case of $gte ) or an empty array where nothing was found. Consistent with $indexOfArray you only want the first match, which is done with another wrapping $arrayElemAt on the output for the 0 position.

Since the result could be an omitted value ( which is what happens by $arrayElemAt: [[], 0] ) then you use [$ifNull][8] to test the result ans pass a two element array back with a -1 as the second element in the case where the output was not defined. In either case that "paired" array has the second element ( index 1 ) extracted again via $arrayElemAt in order to get the first matched index of the condition.

Of course since you want to refer to that whole expression, it just reads a little cleaner in the end within a $let, but that is optional as you can "inline" with the $ifNull if wanted.

So it is possible, it's just a little more involved than placing a range expression inside of $indexOfArray.

Note that any expression which actually returns a single value for equality match is just fine. But since operators like $gte return a boolean, then that would not be equal to any value in the array, and thus the sort of processing with $filter and then extraction is what you require.