How to make MongoDB find nested json return as an array?

Question

I have this data:

{
    "_id" : ObjectId("5461e16ee7caf96f8f3584a2"),
    "num_marcacao" : "100",
    "sexo" : "Fêmea",
    "idade" : "20",
    "bigdata" : {
        "abortos" : [ 
            {
                "data_aborto" : "2014-11-11",
                "causa_aborto" : "Aborto causa 1"
            }, 
            {
                "data_aborto" : "2014-09-01",
                "causa_aborto" : "Aborto causa 2"
            }
        ],
        "crias" : [ 
            ObjectId("5461e16ee7caf96f8f3584a2")
         ]
     }
}
{
    "_id" : ObjectId("5461e1cae7caf96f8f3584a4"),
    "num_marcacao" : "200",
    "sexo" : "Fêmea",
    "bigdata" : {
       "crias" : [ 
           ObjectId("5461e1f3e7caf96f8f3584a5"), 
           ObjectId("5461e760e7caf96f8f3584a6")
       ]
    }
}

Using the following distinct function I get one result

db.animal.distinct('_id', {'bigdata.crias':{$exists:true}}

Result:

{
    "0" : ObjectId("5461e16ee7caf96f8f3584a2"),
    "1" : ObjectId("5461e1cae7caf96f8f3584a4")
}    

Now I want to get the array that is in bigdata.crias like the result of the distinct query. I'm trying to do like this:

db.animal.find(
    {
        $and: [
            {'num_marcacao': '200'},
            {'bigdata.crias':{$exists: true}}
        ]
    },
    {
        'bigdata.crias': true,
        '_id': false
    }
)

But the result is not like the one I need. This is what it's returning:

{
    "bigdata" : {
        "crias" : [ 
            ObjectId("5461e1f3e7caf96f8f3584a5"), 
            ObjectId("5461e760e7caf96f8f3584a6")
        ]
    }
}

And I need

{
    "0" : ObjectId("5461e1f3e7caf96f8f3584a5"),
    "1" : ObjectId("5461e760e7caf96f8f3584a6")
} 

Answer 1

Anyhow. MongoDB does not generally do this from either the .find() or .aggregate() methods or anything general around them. Only the .distinct() method invokes a special form where the result given is "truly" just an array of the specified "key" to be distinct on.

You can always "inspect" the object returned and just use the array element in the structure. You can also specify a "query" argument to the .distinct() command method in the first place:

db.collection.distinct(
    "bigdata.crias",
    { 
        "bigdata.crias": { "$exists": true },
        "num_marcacao": "200"
    }
);

Where you also see your $and argument is redundant. All MongoDB query arguments are an "and" implementation by default. You don't need this unless you are specifying "more than one" condition on the same "field name". That would result in an invalid object by breaking the basic "hash/map" "unique key" rule, and which is why and "array" is used for this form to keep it valid.