mongodb - How to return array index with $indexOfArray -


i have following document structure in mongo:

{     "participantidentities": [         {             "player": {                 "summonername": "summ1",                 "otherdata": "whatever"             },             "participantid": 1         },         {             "player": {                 "summonername": "summ2",                 "otherdata": "whatever2"             },             "participantid": 2         }     ] }

and trying return index of 1 of elements, can pass next part of pipeline it's throwing error.

code:

db.match.aggregate(     [         {             "$project": {                 "matchedindex": {                     "$indexofarray": [                         "$participantidentities", {"$player.summonername": {$eq: "summ2"}}                     ]                 }             }         }     ] )

error:

"ok" : 0, "errmsg" : "unrecognized expression '$participantidentities.player.summonername'", "code" : 168, "codename" : "invalidpipelineoperator"

i'm not sure i'm doing wrong, i'm not sure if querying current array's elements supported might issue?

in example above, i'd want return second element's array index (where summonername summ2) i.e. {"matchedindex": 1}. doing wrong?

if look @ usage tells you. "array" first argument , second argument "search expression" must produce value equality check.

so rather write expression "test element equality" instead "point element in array test". means notating "$participantidentities.player.summonername" in order test against "name" properties converted array:

db.match.aggregate([  { "$project": {    "matchedindex": {      "$indexofarray": [        "$participantidentities.player.summonername",        "summ2"      ]    }  }} ])

the notation takes source array , makes appear follows comparison:

[ "summ1", "summ2" ]

or if prefer, think of above javascript equivalent idiom, using .map() , indexof()

participantidentities.map( p => p.player.summonername ).indexof("summ2")

which same thing practical standpoint. 

"$participantidentities.player.summonername"

is "shorthand" using $map operator of mongodb.

and in comparison, since "second index" n-1 or 1 match, value returned.


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