Create array with query MongoDb

Question

I have this Collection

{
    "_id" : ObjectId("55555555ffffff000010200a"),  
    "name" : "foo"
}

{
    "_id" : ObjectId("55555555ffffff000010200e"),
    "name" : "bar"
}

{
    "_id" : ObjectId("55555555ffffff000010200f"),    
    "name" : "baz"
}


{
    "_id" : ObjectId("55555555ffffff000010200b"),
    "name" : "biz"
}

and I want an array ids

I did this

db.mycollection.find({}, {_id: 1})

and return

{
    "_id" : ObjectId("55555555ffffff000010200a")
}

{
    "_id" : ObjectId("55555555ffffff000010200e")
}

{
    "_id" : ObjectId("55555555ffffff000010200f")
}

{
    "_id" : ObjectId("55555555ffffff000010200b")
}

{
    "_id" : ObjectId("55555555ffffff000010200c")
}

{
    "_id" : ObjectId("55555555ffffff0000103111")
}

Can I have an array? id['55555555ffffff0000103111','55555555ffffff0000103111','555555555ffffff0000103111','55555555ffffff0000103111']

Answer 1

Option 1: If an array of subdocuments is acceptable, you can just append .toArray() to the find query.

> db.test.find({}, {_id:1}).toArray()
[
        {
                "_id" : ObjectId("53270b12d111b9bf595f4270")
        },
        {
                "_id" : ObjectId("53270b14d111b9bf595f4271")
        },
        {
                "_id" : ObjectId("53270b16d111b9bf595f4272")
        }
]

Option 2: Another option is to use the aggregation framework.

> db.test.aggregate([{$group:{_id:null, ids:{$push:"$_id"}}}, {$project:{_id:0, ids:1}}])
{
        "result" : [
                {
                        "ids" : [
                                ObjectId("53270b12d111b9bf595f4270"),
                                ObjectId("53270b14d111b9bf595f4271"),
                                ObjectId("53270b16d111b9bf595f4272")
                        ]
                }
        ],
        "ok" : 1
}

Option 3: Or you can use forEach in the shell like this:

> var myIds = new Array()
> db.test.find({},{_id:1}).forEach(function(myDoc){myIds.push(myDoc._id.str)})
> myIds
[
        "53270b12d111b9bf595f4270",
        "53270b14d111b9bf595f4271",
        "53270b16d111b9bf595f4272"
]