MongoDB fields limitation in array

Question

I am looking for a way - and dont even now if this is possible - just to return a part of a list saved in mongodb.

Lets have a look in my currently document:

{
    _id : 'MyId',
    name : 'a string',
    conversations : [
        {
            user : 'Mike',
            input : 'Some input'
        },
        {
            user : 'Stephano',
            input : 'some other input'
        }
    ]

}

What I now want to do is smth like this:

var myOutput;
myOutput = db.my_collection.find(
{
    _id : 'MyId',
    'conversations.user' : 'Mike'
}, {
    _id : 1,
    name : 1,
    conversations : {
        $where : {
            user : 'Mike'
        }
    }
});

Goal is it just to get back the conversation array item where user has the value Mike.

Is this still possible in MongoDB ? didn't found any reference in the documentation for the field limitations in mongoDB.

the Mongodb documentation has multiple examples on how to use find. Some are similar to what you need, http://docs.mongodb.org/manual/reference/method/db.collection.find/ — faljbour, Feb 21 '15 at 19:12
I dont need a find example - how to find a document where this user is set is in my example - I need a way to limitate the returned value (just return a part of the array) — TJR, Feb 21 '15 at 19:14

score 2 · Accepted Answer · answered Feb 22 '15 at 01:07

Use the $ positional operator in a projection:

> db.my_collection.find({ "_id" : "MyId", "conversations.user" : "Mike" },
                      { "_id" : 1, "name" : 1, "conversations.$" : 1 })
{
    "_id" : 'MyId',
    "name" : 'a string',
    "conversations" : [
        { "user" : 'Mike', "input" : 'Some input' }
    ]
}

This projects only first matching array element.

styvane · Answer 2 · 2015-03-21T20:29:10.780

1

Are you aware of the aggregation pipeline?

db.my_collection.aggregate([
    { "$match": { "_id": "MyId"}}, { "$unwind": "$conversations"}, 
    { "$match": {"conversations.user": "Mike"}}
])

Output

{ 
    "_id" : "MyId", 
    "name" : "a string", 
    "conversations" : 
    { 
        "user" : "Mike", 
        "input" : "Some input" 
    } 
}

edited Mar 21 '15 at 20:29

answered Feb 21 '15 at 19:09

styvane

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MongoDB fields limitation in array

2 Answers2