get matching array element from document in mongodb

Question

Assuming I have this documents in mongodb:

{
    "_id" : ObjectId("512e28984815cbfcb21646a7"),
    "myList" : [
        {
            "a" : true,
            "b" : mydata1
        },
        {
            "a" : true
            "b" : mydata2
        },
        {
            "a" : false
            "b" : mydata3
        },
        {
            "a" : false
            "b" : mydata4
        },
        {
            "a" : true
            "b" : mydata5
        }
    ]
}

I want to get all array cells that have "a:true" in them.

I tried mongodb find(), but it only return single first match, which is a problem.

I ended up using $filter because I do not want to use $unwind (performance issues).

trying to execute this line, doesn't work:

.aggregate([
{$match: {_id: NUUID("d75dcc8d-ecb2-4d23-99d5-347bc99cfeec")}}, 
{$project:
    {
        myList: 
        {
            $filter: {
                input: '$myList',
                as: 'myElement',
                cond: {$eq: [{'$$myElement.a': true}]}
        }}
    }
 }
 ])

But this doesn't seems to work, it says that $$ is not recognized. I wander what am I missing and also, why can't find() be used? it sound so basic to ask for some elements in array using a project.

NUUID does not exist, what r u talking about? UUID(<string>) instead works fine. Also $eq:[{"$$a":"b"}] should be $eq:[{"$$a","b"}] with a comma between the things you compare — user12582392
– user12582392, Commented Jan 7, 2021 at 22:35

R2D2 · Accepted Answer · 2021-01-07 22:27:02Z

1

For me is working (sharded cluster version 4.4.3):

mongos> db.e.find()
{ "_id" : ObjectId("512e28984815cbfcb21646a7"), "myList" : [ { "a" : true, "b" : "mydata1" }, { "a" : true, "b" : "mydata2" }, { "a" : false, "b" : "mydata3" }, { "a" : false, "b" : "mydata4" }, { "a" : true, "b" : "mydata5" } ] }
{ "_id" : ObjectId("512e28984815cbfcb21646a8"), "myList" : [ { "a" : true, "b" : "mydata1" }, { "a" : true, "b" : "mydata2" }, { "a" : false, "b" : "mydata3" }, { "a" : false, "b" : "mydata4" }, { "a" : true, "b" : "mydata5" }, { "a" : true, "b" : "mydata6" } ] }

mongos> db.e.aggregate({$project: { myList: { $filter:{  input:"$myList" , as:"element" , cond: {$eq:[ "$$element.a",true  ] }               }   }         }})
{ "_id" : ObjectId("512e28984815cbfcb21646a7"), "myList" : [ { "a" : true, "b" : "mydata1" }, { "a" : true, "b" : "mydata2" }, { "a" : true, "b" : "mydata5" } ] }
{ "_id" : ObjectId("512e28984815cbfcb21646a8"), "myList" : [ { "a" : true, "b" : "mydata1" }, { "a" : true, "b" : "mydata2" }, { "a" : true, "b" : "mydata5" }, { "a" : true, "b" : "mydata6" } ] }
mongos>

answered Jan 7, 2021 at 22:27

R2D2

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1 Comment

toto Over a year ago

I used robomongo for nuuid conversion, but that doesn't relate to the question, in any case the equaliy was the issue thanks for the example kiko075

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