Mongo conditional for "key doesn't exist"?

Question

I want to find a document using a conditional if the key == None or if the key doesn't exist. Something like this:

myDoc = self.request.root.db.myDocs.find_one({
                          '$or': [
                              {'myKey' : $doesNotExist } ,
                              {'myKey' : None }
                            ]
                    })

I'd also like to be able to find a document just by a missing key like this:

myDoc = self.request.root.db.myDocs.find_one( {'myKey' : $doesNotExist } )

How can I accomplish this?

score 29 · Accepted Answer · answered Mar 18 '13 at 21:31

For "if key exists" checks, using a .find() is significantly faster than find_one().

Single document: cursor = db.myDocs.find({"mykey": {"$exists": True}}).limit(1)

Multiple documents: cursor = db.myDocs.find({"mykey": {"$exists": True}})

if cursor.count() > 0:
    keyExists = True
else:
    keyExists = False

score 19 · Answer 2 · edited Jul 10 '15 at 12:19

19

You can test for a key not existing with:

db.myDocs.find_one({'myKey': { '$exists': False }})

Mongo documentation about the $exists operator

edited Jul 10 '15 at 12:19

Etienne

655
3
15

answered Aug 19 '12 at 22:49

Joe Day

6,965
4
25
26

1

I'm using MongoDB compass (1.24.6), and "False" didn't work but this worked: `{"myKey": {$exists: 0}}` – MD004 Mar 16 '22 at 23:02

Mongo conditional for "key doesn't exist"?

2 Answers2