Question

Check out the following query:

 db.models.findOne({ "value.userId": { $ne: null } }, { "value.userId":1}).value.userId == null

This says:

1. Find a record that has a non-null user id field
2. Return the user id in the result
3. Compare that value to null

Since we’re returning only records without null values, comparing the result of that to null should always return false. However, this always returns true.

If I just do the .findOne(…) and print that instead of doing the comparison, I get:

{ "_id": 4, "value": { "userId": null }

Anyone have a clue what’s going on here?

EDIT: the “type” of this field is apparently 6 – not 10, which is what a null should be.
EDIT2: apparently type 6 is “undefined” – not sure why it prints null…

Answer 1

Well the issue here is when you try to print an undefined value, MongoDB prints null rather than issuing you an error.

See this example: –

> db.foo.save({"amount": undefined})
> db.foo.find({"amount": {$ne: null}})
{ "_id" : ObjectId("50d0d3a1511dd1035f01c636"), "amount" : null }

So, as you are saying, the type of userId is 6, which is for undefined, so it prints null. Also, it is $ne: null, so it is matched too.

See this link: – http://realprogrammer.wordpress.com/2012/11/04/null-undefined-nan-and-missing-property-goto-considered-harmful-part-2/

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However, if you see the same behaviour with null, you will get the desired result: –

> db.foo.save({"amount": null})
> db.foo.findOne({"amount": {$ne: null}}) 
null