How can I get all the doc ids in MongoDB?

Question

How can I get an array of all the doc ids in MongoDB? I only need a set of ids but not the doc contents.

Answer 1

You can do this in the Mongo shell by calling map on the cursor like this:

var a = db.c.find({}, {_id:1}).map(function(item){ return item._id; }) 

The result is that a is an array of just the _id values.

The way it works in Node is similar.

(This is MongoDB Node driver v2.2, and Node v6.7.0)

db.collection('...')   .find(...)   .project( {_id: 1} )   .map(x => x._id)   .toArray(); 

Remember to put map before toArray as this map is NOT the JavaScript map function, but it is the one provided by MongoDB and it runs within the database before the cursor is returned.

Answer 2

One way is to simply use the runCommand API.

db.runCommand ( { distinct: "distinct", key: "_id" } ) 

which gives you something like this:

{     "values" : [         ObjectId("54cfcf93e2b8994c25077924"),         ObjectId("54d672d819f899c704b21ef4"),         ObjectId("54d6732319f899c704b21ef5"),         ObjectId("54d6732319f899c704b21ef6"),         ObjectId("54d6732319f899c704b21ef7"),         ObjectId("54d6732319f899c704b21ef8"),         ObjectId("54d6732319f899c704b21ef9")     ],     "stats" : {         "n" : 7,         "nscanned" : 7,         "nscannedObjects" : 0,         "timems" : 2,         "cursor" : "DistinctCursor"     },     "ok" : 1 } 

However, there's an even nicer way using the actual distinct API:

 var ids = db.distinct.distinct('_id', {}, {}); 

which just gives you an array of ids:

[     ObjectId("54cfcf93e2b8994c25077924"),     ObjectId("54d672d819f899c704b21ef4"),     ObjectId("54d6732319f899c704b21ef5"),     ObjectId("54d6732319f899c704b21ef6"),     ObjectId("54d6732319f899c704b21ef7"),     ObjectId("54d6732319f899c704b21ef8"),     ObjectId("54d6732319f899c704b21ef9") ] 

Not sure about the first version, but the latter is definitely supported in the Node.js driver (which I saw you mention you wanted to use). That would look something like this:

db.collection('c').distinct('_id', {}, {}, function (err, result) {     // result is your array of ids })