How to find length of embedded nested array in mongodb?

Question

How to find length of embedded nested array in mongodb? For example, I have the below collection in my mongoDB. The loc field contains coordinates field which is an array and again it contains another array which is in 0th index. How can I find length of loc.corrdinates[0] ? I tried to do this with using aggregation pipeline but it gives syntax error:

db.atolldata.aggregate([
                        {$project:{_id:0,
                                   fileName:1,
                         sizeOfArray:{$size:"$loc.coordinates[0]"}}}]);

.

Here is my collection

db.mydb.findOne();

{
    "_id" : ObjectId("54ccb36fe4b0ff85abfee006"),
    "_class" : "com.inn.signaleye.model.AtollData",
    "rsrp" : "-124",
    "index" : 1,
    "rgb" : [
        192,
        157,
        0
    ],
    "loc" : {
        "type" : "Polygon",
        "coordinates" : [
            [
                [
                    91.5254213240325,
                    26.225096
                ],
                [
                    91.5254099392226,
                    26.225999
                ],
                [
                    91.52591034644685,
                    26.226004
                ],
                [
                    91.52592172739666,
                    26.225101
                ],
                [
                    91.5254213240325,
                    26.225096
                ]
            ]
        ]
    },
    "fileName" : "RSRP_ZOne46N_20m_SCFT.shp"
}

BatScream's answer is good, but are you actually trying to get the length of coordinates in a single document, or in all documents? If the former, it may be easier to just get that document by ID and compute its length in your application. — i80and
– i80and, Commented Feb 2, 2015 at 17:40

BatScream · Accepted Answer · 2015-02-01 08:54:16Z

3

Make use of the $unwind stage, to flatten the coordinates array, and then find the size in the $project stage.

db.atolldata.aggregate([
{$unwind:"$loc.coordinates"},
{$project:{"_id":0,"fileName":1,
           "sizeOfArray":{$size:"$loc.coordinates"}}}
])

edited Feb 1, 2015 at 8:54

answered Feb 1, 2015 at 8:28

BatScream

19.7k4 gold badges54 silver badges68 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Mr. Potato · Accepted Answer · 2022-07-31 09:28:00Z

0

You might want to create a field with the size of the array to work with it:

{$set : {arraySize: {$size: {$arrayElemAt : ['$loc.coordinates', 0]}}}}

Remember all documents have to include the loc field or the aggregation will fail (so you have to filter these documents first with $match).

answered Jul 31, 2022 at 9:28

Mr. Potato

112 bronze badges

Collectives™ on Stack Overflow

How to find length of embedded nested array in mongodb?

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related