MongoDB Null Query Anomaly

Question

Check out the following query:

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

This says:

Find a record that has a non-null user id field
Return the user id in the result
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...

Strange this is. Because, it is giving expected result for me. Are you sure this is working this way? — Rohit Jain
– Rohit Jain, Commented Dec 18, 2012 at 20:01
Positive...just ran this: db.models.findOne({ "value.userId": { $ne: null } }).value.userId, and it returns null. — Jerod Venema
– Jerod Venema, Commented Dec 18, 2012 at 20:03

Rohit Jain · Accepted Answer · 2012-12-18 20:45:17Z

3

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/

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

edited Dec 18, 2012 at 20:45

answered Dec 18, 2012 at 20:39

Rohit Jain

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11 Comments

Jerod Venema Over a year ago

Well now that's just annoying.

Rohit Jain Over a year ago

@jvenema. Well, that may seem annoying now, but actually this feature is very useful, when you accidentally try to print an undefined field, in which case you won't get errors. That is how MongoDB is designed to be.

Jerod Venema Over a year ago

I get that it's designed to "be that way", but the behaviour should be consistent - either print and filter nulls and undefined values the same way (both), or don't - but don't print one way, and filter another...

Rohit Jain Over a year ago

@jvenema.. Actually a field is undefined either by giving it explicit value of undefined, or when it is not there at all in the document. So, of course you cannot filter on the basis of a field that is not there, i.e., a non-existence or undefined field cannot be equal to null.

Jerod Venema Over a year ago

Sorta - in the find() call you can't filter on null == undefined, but in the console they can be compared (see my original question - comparing to null is true), and they obviously are considered to be equal "enough" for the purposes of writing to the output - otherwise you would see "undefined" and not "null" as the output of the script. The problem is inconsistency - in some cases "null" cannot be compared and is not the same as "undefined", but in others it is.

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