MongoDB aggregate by field exists

Question

I have a hard time believing this question hasn't been asked and answered somewhere already, but I can't find any trace of it.

I have a MongoDB aggregation query that needs to group by a boolean: the existence of another field.

For example let's start with this collection:

> db.test.find() { "_id" : ObjectId("53fbede62827b89e4f86c12e"),   "field" : ObjectId("53fbede62827b89e4f86c12d"), "name" : "Erik" } { "_id" : ObjectId("53fbee002827b89e4f86c12f"), "name" : "Erik" } { "_id" : ObjectId("53fbee092827b89e4f86c131"),   "field" : ObjectId("53fbee092827b89e4f86c130"), "name" : "John" } { "_id" : ObjectId("53fbee122827b89e4f86c132"), "name" : "Ben" } 

2 documents have "field", and 2 don't. Note that each value of "field" may be different; we just want to group on its existence (or non-nullness works for me too, I don't have any null values stored).

I've tried using $project, but $exists doesn't exist there, and $cond and $ifNull haven't helped me. The field always appears to exist, even when it doesn't:

> db.test.aggregate(   {$project:{fieldExists:{$cond:[{$eq:["$field", null]}, false, true]}}},   {$group:{_id:"$fieldExists", count:{$sum:1}}} ) { "_id" : true, "count" : 4 } 

I would expect the following much simpler aggregate to work, but for some reason $exists isn't supported in this way:

> db.test.aggregate({$group:{_id:{$exists:"$field"}, count:{$sum:1}}}) assert: command failed: {   "errmsg" : "exception: invalid operator '$exists'",   "code" : 15999,   "ok" : 0 } : aggregate failed Error: command failed: {   "errmsg" : "exception: invalid operator '$exists'",   "code" : 15999,   "ok" : 0 } : aggregate failed     at Error (<anonymous>)     at doassert (src/mongo/shell/assert.js:11:14)     at Function.assert.commandWorked (src/mongo/shell/assert.js:244:5)     at DBCollection.aggregate (src/mongo/shell/collection.js:1149:12)     at (shell):1:9 2014-08-25T19:19:42.344-0700 Error: command failed: {   "errmsg" : "exception: invalid operator '$exists'",   "code" : 15999,   "ok" : 0 } : aggregate failed at src/mongo/shell/assert.js:13 

Does anyone know how to get the desired result from a collection like this?

Expected result:

{ "_id" : true, "count" : 2 } { "_id" : false, "count" : 2 } 

Answer 1

I solved the same problem just last night, this way:

> db.test.aggregate({$group:{_id:{$gt:["$field", null]}, count:{$sum:1}}}) { "_id" : true, "count" : 2 } { "_id" : false, "count" : 2 } 

See http://docs.mongodb.org/manual/reference/bson-types/#bson-types-comparison-order for a full explanation of how this works.