Extracting particular element in MongoDb in Nested Arrays

Question

{
"_id" : 123,
"someKey" : "someValue",
"someArray" : [ 
    {
        "name" : "name1",
        "someNestedArray" : [ 
            {
                "name" : "value_1",
                "fame" : [ 
                    {
                        "goal" : "goal_1"
                    }
                ]
            }, 
            {
                "name" : "value_2",
                "fame" : [ 
                    {
                        "goal" : "goal_2"
                    }
                ]
            }
        ]
    }
]
}

I does the query like:

db.getCollection('city').find({"someArray.0.someNestedArray.1.fame.0.goal":"goal_2"},{"someArray.0.someNestedArray.":1})

But get the output as:

{
"_id" : 123,
"someArray" : [ 
    {}
]
}

What is the query for getting the output as:

{
"fame" : [ 
                {
                    "goal" : "goal_2"
                }
            ]
}

Thanks in advance. Also tell how to find a value in nested array when one don't know the indexes. The query I have done specifies the array element position (someArray.0.someNestedArray.1.fame.0.goal)

check [mongodb positional operator](https://docs.mongodb.com/manual/reference/operator/update/positional/) — Jyothi Babu Araja, Feb 11 '17 at 12:55
The positional $ operator cannot be used for queries which traverse more than one array, such as queries that traverse arrays nested within other arrays, because the replacement for the $ placeholder is a single value. — Pranav Anand, Feb 11 '17 at 16:26

McGrady · Accepted Answer · 2017-02-13T03:34:21.380

You can just use this following query:

db.test.find(
    {'someArray.someNestedArray.fame.goal':'goal_1'},
    {"someArray.someNestedArray.fame.goal":1,"_id":0}
)

And you will get the document:

{
    "someArray" : [ 
        {
            "someNestedArray" : [ 
                {
                    "fame" : [ 
                        {
                            "goal" : "goal_1"
                        }
                    ]
                }, 
                {
                    "fame" : [ 
                        {
                            "goal" : "goal_2"
                        }
                    ]
                }
            ]
        }
    ]
}

Also you can try to use $elemMatch

The $elemMatch operator matches documents that contain an array field with at least one element that matches all the specified query criteria.

db.test.find(
{
    'someArray':{
        $elemMatch:{
            'someNestedArray':{
                $elemMatch:{
                    'fame':{ $elemMatch: {"goal" : "goal_2" } }
                    }
                }
        }
    }
},{"someArray.someNestedArray.fame.goal":1}
);

You can try to use $unwind and $replaceroot to return subdocument,and use the new $filter aggregation operator to filter an array during projection.

Hope this helps.

Extracting particular element in MongoDb in Nested Arrays

1 Answers1

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