query document nested multiarray-array mongodb elemMatch

Question

I have this documents:

//document 1
{
    info : [
        {
            id  : 100,
            field : {                   
                a : 1,
                b : 2
            }
        },
        {
            id  : 200,
            field : {                   
                a : 3,
                b : 4
            }
        },
        {
            id  : 300,
            field : {                   
                a : 5,
                b : 6
            }
        }
    ]
},
//document 2
{
    info : [
        {
            id  : 400,
            field : {                   
                a : 7,
                b : 8
            }
        },
        {
            id  : 500,
            field : {                   
                a : 9,
                b : 10
            }
        }
    ]
}

I need to find the id of the subdocument with the values field.a = 7 and field.b = 8 , that means the id value is 400.

What i have tried is $elemMatch but I can't get the result.

My attemps :

attemp 1:

db.mycollection.findOne({info : {$elemMatch : {  'field.$.a':7,'field.$.b':8   } } });

attemp 2:

db.mycollection.findOne({info:{$elemMatch:{$elemMatch:{'field.$.a':7,'field.$.b':8,}}}});

attemp 3:

db.mycollection.findOne({info:{$elemMatch:{$elemMatch:{'field.a.$':7,'field.b.$':8,}}}});

attemp 4:

db.mycollection.findOne({info:{$elemMatch:{'field.$.a':7,'field.$.b':8,}}});

you'll have to use the aggregation framework to get the subdocument's id — Abdullah Rasheed
– Abdullah Rasheed, Commented Nov 18, 2015 at 21:04
try db.mycollection.findOne({info : {$elemMatch : {field:{ 'a':7,'b':8 }} } }); — hecnabae
– hecnabae, Commented Nov 18, 2015 at 21:04
this actually works, i just have tu use projection db.mycollection.findOne({info : {$elemMatch : {field:{ 'a':7,'b':8 }} } }, { id:1, info : {$elemMatch : {field:{ 'a':7,'b':8 }} } });, thanks @hecnabae — Luis Felipe Garcia
– Luis Felipe Garcia, Commented Nov 18, 2015 at 21:24

Blakes Seven · Accepted Answer · 2015-11-19 04:48:41Z

2

The $elemMatch operator works like a "mini query" against the specified array element it is acting on, so arguments go inside. Also the positional $ operator here is a propery of "projection" and not the query document itself, so this is a separate element:

db.mycollection.find(
    {
        "info": {
            "$elemMatch": { "field.a": 7 , "field.b": 8 }
        }
    },
    { "info.$": 1 }
)

Which both matches the document containing the matched element, and then only returns the matched element due to the projection:

{
        "_id" : ObjectId("564d52979f28c6e0feabceee"),
        "info" : [
                {
                        "id" : 400,
                        "field" : {
                                "a" : 7,
                                "b" : 8
                        }
                }
        ]
}

answered Nov 19, 2015 at 4:48

Blakes Seven

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