MongoDB not equal to

Use $ne -- $not should be followed by the standard operator:

An examples for $ne, which stands for not equal:

use test
switched to db test
db.test.insert({author : 'me', post: ""})
db.test.insert({author : 'you', post: "how to query"})
db.test.find({'post': {$ne : ""}})
{ "_id" : ObjectId("4f68b1a7768972d396fe2268"), "author" : "you", "post" : "how to query" }

And now $not, which takes in predicate ($ne) and negates it ($not):

db.test.find({'post': {$not: {$ne : ""}}})
{ "_id" : ObjectId("4f68b19c768972d396fe2267"), "author" : "me", "post" : "" }

From the Mongo docs:

The $not operator only affects other operators and cannot check fields and documents independently. So, use the $not operator for logical disjunctions and the $ne operator to test the contents of fields directly.

Since you are testing the field directly $ne is the right operator to use here.

Edit:

A situation where you would like to use $not is:

db.inventory.find( { price: { $not: { $gt: 1.99 } } } )

That would select all documents where:

• The price field value is less than or equal to 1.99 or the price
• Field does not exist

If you want to do multiple $ne then do

db.users.find({name : {$nin : ["mary", "dick", "jane"]}})

Use $ne instead of $not

http://docs.mongodb.org/manual/reference/operator/ne/#op._S_ne

db.collections.find({"name": {$ne: ""}});