mongoDB: get field value of an array object

Question

I want to get the value of the field 30 in the object (in the array test) with the id ePce6fBAHx9KeKjuM.

{
    "_id" : "nAwt3b76c24mZfqxz",
    "title" : "test",
    "test" : [
        {
            "4" : false,
            "15" : false,
            "30" : false,
            "75" : true,
            "id" : "ePce6fBAHx9KeKjuM"
        }
    ]
}

So this result would be false

I tried something like

var result = Collection.findOne({_id: 'nAwt3b76c24mZfqxz'}).test;

But this would give me the complete array. But I need the selected object and only a selected field of this object (ie. 30).

Alex Blex · Accepted Answer · 2016-01-26 11:04:16Z

2

test is just a JS array. Use normal array syntax to access its elements:

var result = Collection.findOne({_id: 'nAwt3b76c24mZfqxz'}).test["30"];

EDIT:

To retrieve the whole object with only 1 element of the array use projection, as of zangw's answer. Following your comment to test element itself:

db.getCollection('a').find(
    // put your nested document's condition instead of `$exists`
    {_id: 'nAwt3b76c24mZfqxz', test:{ $elemMatch: { "30": {$exists: true}}}},
    // add other fields you like to retrieve, e.g. "title":1
    {"test.30":1}
)

edited Jan 26, 2016 at 11:04

answered Jan 26, 2016 at 10:00

Alex Blex

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2 Comments

user3142695 Over a year ago

But this doesn't consider, that test is an array, and I need the result of the object with the given id.

Alex Blex Over a year ago

It would be nice to have an example of expected result, as it is not quite clear what you are trying to retrieve.

zangw · Accepted Answer · 2016-01-26 10:36:07Z

0

Try this one

Collection.find({{_id: 'nAwt3b76c24mZfqxz'}}, {'test.30': 1, 'test.id': 1});

To select the whole test array as following without _id

Collection.find({{_id: 'nAwt3b76c24mZfqxz'}}, {'test': 1, '_id': 0});

edited Jan 26, 2016 at 10:36

answered Jan 26, 2016 at 10:16

zangw

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3 Comments

user3142695 Over a year ago

But I need the exact object of the array test (id = ePce6fBAHx9KeKjuM).

zangw Over a year ago

@user3142695, add {test : 1} to select this field out.. Is it meet your requirement? or am I missing something?

user3142695 Over a year ago

I'm not quite sure, but this just selects the field generally. I have to check the defined object in this array. So if there isn't an object with the given id, the result should be empty/false.

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