How to query an array of _id in MongoDB?

Question

I have collection Collection1 and I need to fetch an array like: [id1, id2, id3, ...] (array which consist of _id`s for every element in this collection). Is there any way to do this query with MongoDB tools ? Thank you!

Answer 1

You will have to use cursor.toArray() function on result of find having a projection document projecting only _id i.e ObjectId() value

MongoDB Enterprise > db.users.find().pretty()
{
        "_id" : ObjectId("570e1d465a44f125ef156791"),
        "name" : "Name1",
        "age" : 22,
        "gender" : "M"
}
{
        "_id" : ObjectId("570e1e1d5a44f125ef156792"),
        "name" : "Name2",
        "age" : 21,
        "gender" : "F"
}
{
        "_id" : ObjectId("570e1e485a44f125ef156793"),
        "name" : "Name3",
        "age" : 22,
        "gender" : "M"
}
{
        "_id" : ObjectId("570e28d45a44f125ef156794"),
        "name" : "Name4",
        "age" : 21,
        "gender" : "F"
}
{ "_id" : 123 }
MongoDB Enterprise > db.users.find({}, {_id:1}).toArray()
[
        {
                "_id" : ObjectId("570e1d465a44f125ef156791")
        },
        {
                "_id" : ObjectId("570e1e1d5a44f125ef156792")
        },
        {
                "_id" : ObjectId("570e1e485a44f125ef156793")
        },
        {
                "_id" : ObjectId("570e28d45a44f125ef156794")
        },
        {
                "_id" : 123
        }
]
MongoDB Enterprise >

As an addition you can also use:

MongoDB Enterprise > db.users.distinct("_id")
[
        123,
        ObjectId("570e1d465a44f125ef156791"),
        ObjectId("570e1e1d5a44f125ef156792"),
        ObjectId("570e1e485a44f125ef156793"),
        ObjectId("570e28d45a44f125ef156794")
]
MongoDB Enterprise >