MongoDB aggregate by field exists

问题:

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 }

回答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.

回答2:

The $exists operator is a "query" operator, so it is used basically to "filter" results rather than identify a logical condition.

As a "logical" operator the aggregation framework supports the $ifNull operator. This returns the field value where it exists or the alternate supplied value where it does not or otherwise evaluates to null

db.test.aggregate([
    { "$group": {
        "_id": { "$ifNull": [ "$field", false ] },
        "count": { "$sum": 1 }
    }}
])

But of course, even that is not a "true/false" comparison, so unless you actually want to return the actual value of the field where it is present, then you are probably better off with a $cond statement much like you have:

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

Where $ifNull can be very useful is in replacing not existent array fields that would otherwise cause an error using $unwind. You can then do something like return a single element or empty array so this does not cause problems in the rest of your pipeline processing.

回答3:

I solved it with checking for undefined

$ne : [$var_to_check, undefined]

or

$ne:  [ { $type : "$var_to_check"}, 'missing'] }

This returns true if the var is defined