How to concat numeric and string value in mongodb?

Question

I need to select a value from an array in a json file in mongodb.

The json looks like this:

{
    "key" : {
        "subkey" : "value",
        "subkey1" : "value",
        "subkey2" : "value",
        "firstArray" : [ 
            NumberLong(1234),
            "secondIndex"
        ]
    }
}

I'm trying to select firstIndex, my query looks like this:

db.getCollection('table_name').aggregate([{
    $project: {
        columnName: {
            $concat: [{
                $substr: ["$key.firstArray[0]"],
                "hello world"
            }
            ]
       }
   }
}])

But this returns an empty string. I don't understand why.

The other thing I tried was using $arrayElemAt, which looks like:

db.getCollection('table_name').aggregate([{
    $project: {
        columnName: {
            $concat: [{
                $arrayElemAt: ["$key.firstArray", 0],
                "hello world"
            }]
       }
   }
}])

But this returns a concat only supports strings, not NumberLongs.

No concatenation needed here. Try db.getCollection('table_name').aggregate([{ $project: { columnName: { $arrayElemAt: ["$key.firstArray", 0] } } }]) — s7vr
– s7vr, Commented Feb 8, 2018 at 21:41
But I need to append the result onto a string that I want, I guess I didn't specify in the original question, I'm updating it now — user9333933
– user9333933, Commented Feb 9, 2018 at 1:01
Try db.getCollection('table_name').aggregate([{ $project: { columnName: { $concat: [{$substr:[{ $arrayElemAt: ["$key.firstArray", 0] }, 0, -1]}, "hello world"] } } }]) — s7vr
– s7vr, Commented Feb 9, 2018 at 1:10
It's not working for me, robomongo keeps telling me error line 12 unexpected token. Where my arrayelemAt starts. — user9333933
– user9333933, Commented Feb 9, 2018 at 1:32
Please copy paste code from my last comment. Your code has syntax errors. — s7vr
– s7vr, Commented Feb 9, 2018 at 1:33

s7vr · Accepted Answer · 2018-10-22 18:33:42Z

13

You can use $toString from mongo 4.0 version.

db.getCollection('table_name').aggregate([
 {"$project":{
   "columnName":{
     "$concat":[
       {"$toString":{"$arrayElemAt":["$key.firstArray",0]}},
       "hello world"
     ]
   }
 }}
])

You can try below query to $concat long and string value. Use $substr to convert the computed long value to string.

db.getCollection('table_name').aggregate([
 {"$project":{
   "columnName":{
     "$concat":[
       {"$substr":[{"$arrayElemAt":["$key.firstArray",0]},0,-1]},
       "hello world"
     ]
   }
 }}
])

edited Oct 22, 2018 at 18:33

answered Feb 9, 2018 at 1:38

s7vr

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1 Comment

user9333933 Over a year ago

This works, thank you very much, I had my hello world within the concat curly bracket

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