Finding nested array mongodb

Question

So I have a mongodb document looking like this:

[
    {
        "_id": "04",
        "name": "test service 4",
        "id": "04",
        "version": "0.0.1",
        "title": "testing",
        "description": "test",
        "protocol": "test",
        "operations": [
            {
                "_id": "99",
                "oName": "test op 52222222222",
                "sid": "04",
                "name": "test op 52222222222",
                "oid": "99",
                "description": "testing",
                "returntype": "test",
                "parameters": [
                    {
                        "oName": "Param1",
                        "name": "Param1",
                        "pid": "011",
                        "type": "582",
                        "description": "testing",
                        "value": ""
                    },
                    {
                        "oName": "Param2",
                        "name": "Param2",
                        "pid": "012",
                        "type": "58222",
                        "description": "testing",
                        "value": ""
                    }
                ]
            }
        ]
    }
]

I want to be able to find all of the parameters and an individual one but I am not entirely sure how, if I use this:

 collection.find({operations: {$elemMatch: {oid: oid}}}, {"operations.$.parameters": 1}).toArray(function(error, result) {
    if (error) {
        console.log('Error retrieving parameter: ' + error);
        res.send({'error':'An error has occurred'});
    } else {
        // console.log(result);
        res.send(result);
    }
});

the output I get is this:

[
    {
        "_id": "04",
        "operations": [
            {
                "_id": "99",
                "oName": "test op 52222222222",
                "sid": "04",
                "name": "test op 52222222222",
                "oid": "99",
                "description": "testing",
                "returntype": "test",
                "parameters": [
                    {
                        "oName": "Param1",
                        "name": "Param1",
                        "pid": "011",
                        "type": "582",
                        "description": "testing",
                        "value": ""
                    }
                ]
            }
        ]
    }
]

even though the only thing I want is the individual parameter. Is there are any way I can get this done?

chridam · Accepted Answer · 2015-04-14T17:24:00.377

You could use the aggregation framework for this:

var oid = "99",
    pipeline = [
    {
        $match: {
            "operations.oid": oid
        }
    },
    {
        $unwind: "$operations"
    },
    {
        $match: {
            "operations.oid": oid
        }
    },
    {
        $unwind: "$operations.parameters"
    },
    {
        $project: {            
            "parameters": "$operations.parameters"
        }
    }
];

collection.aggregate(pipeline).toArray(function(error, result) {
    if (error) {
        console.log('Error retrieving parameter: ' + error);
        res.send({'error':'An error has occurred'});
    } else {
        // console.log(result);
        res.send(result);
    }
});

Output:

    [ 
        {
            "_id" : "04",
            "parameters" : {
                "oName" : "Param1",
                "name" : "Param1",
                "pid" : "011",
                "type" : "582",
                "description" : "testing",
                "value" : ""
            }
        }, 
        {
            "_id" : "04",
            "parameters" : {
                "oName" : "Param2",
                "name" : "Param2",
                "pid" : "012",
                "type" : "58222",
                "description" : "testing",
                "value" : ""
            }
        }
    ]

EDIT:

To access an individual parameter given a pid value, you can modify the pipeline by adding a $match pipeline stage between the $unwind and $project stages so that you can filter the documents to return the parameter object matching pid :

var oid = "99", 
    pid = "011",
    pipeline = [
        {
            $match: {
                "operations.oid": oid
            }
        },
        {
            $unwind: "$operations"
        },
        {
            $match: {
                "operations.oid": oid
            }
        },
        {
            $unwind: "$operations.parameters"
        },
        // This pipeline stage returns the individual parameters object that matches the given pid value
        {
            $match: { "operations.parameters.pid": pid }
        },
        {
            $project: {            
                "parameters": "$operations.parameters"
            }
        }
    ];

Output:

[ 
    {
        "_id" : "04",
        "parameters" : {
            "oName" : "Param1",
            "name" : "Param1",
            "pid" : "011",
            "type" : "582",
            "description" : "testing",
            "value" : ""
        }
    }
]

This is exactly what I needed, thank you. How would you modify this if you just want an individual parameter? I have access to the pid through req.params.parameters — Santiago Esquivel, Apr 14 '15 at 17:13
@SantiagoEsquivel No worries :-) I have modified my answer to address that part. — chridam, Apr 14 '15 at 17:24
Thats amazing, I cannot thank you enough, I have been struggling with this last bit in my program a lot the last couple days. — Santiago Esquivel, Apr 14 '15 at 17:27
@SantiagoEsquivel That's ok. Bear in mind that the **`$match`**, **`$sort`**, **`$limit`**, and **`$skip`** pipeline operators can take advantage of an index when they occur at the beginning of the pipeline before any of the following aggregation operators: **`$project`**, **`$unwind`**, and **`$group`**. So if you have indexes on the fields used in the `$match` operators above your aggregation query could perform a bit faster. — chridam, Apr 14 '15 at 17:30
Just as a last followup, if I wanted to do an update on any of the given parameters, how should I go about to do this? I tried using $elemMatch but the changes are not reflected, I can make another question in order to explain myself further if needed — Santiago Esquivel, Apr 14 '15 at 17:42
I've just answered a similar query [**here**](http://stackoverflow.com/a/29633993/122005), which I think may be a close match to what you need but then again if that doesn't come close, please feel free to create a new question then we can take it from there. — chridam, Apr 14 '15 at 17:44
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/75220/discussion-between-santiago-esquivel-and-chridam). — Santiago Esquivel, Apr 14 '15 at 17:51

Finding nested array mongodb

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