MongoDB Differentiates Between undefined vs. null

Question

I was checking logic for querying on non-values and noticed when using the mongo shell, it differentiates between undefined and null values.

> use test
> db.test.insert({ a : 1,          b : null,       c : undefined })
> db.test.insert({ a : null,       b : undefined,  c : 1 })
> db.test.insert({ a : undefined,  b : 1,          c : null })

When you query on the collection, you get this:

> db.test.find()
{ "_id" : ObjectId("52d95575c9333565e80ccb22"), "a" : 1, "b" : null, "c" : null }
{ "_id" : ObjectId("52d9557fc9333565e80ccb23"), "a" : null, "b" : null, "c" : 1 }
{ "_id" : ObjectId("52d95586c9333565e80ccb24"), "a" : null, "b" : 1, "c" : null }

When you query on null however, it only retrieves records who was explicitly set to null.

> db.test.find({ a : null })
{ "_id" : ObjectId("52d9557fc9333565e80ccb23"), "a" : null, "b" : null, "c" : 1 }

Is this a bug in MongoDB? How can I properly query for null/ undefined/ non-set fields?

EDIT

So I can query for not-set values with this:

db.test.find({ $or : [ { a : null }, { a : { $exists : false } } ] })

But in this example though, it still only returns the single record:

{ "_id" : ObjectId("52d9557fc9333565e80ccb23"), "a" : null, "b" : null, "c" : 1 }

Anyone know why/ how MongoDB differentiates between undefined and null? Is this an issue with how the data was entered in MongoDB?

Answer 1

If you want to return a document where a field exists AND is not null, use { a : {$ne : null}}

Undefined and null values are different, but the shell shows them both as null - https://jira.mongodb.org/browse/SERVER-6102

Answer 2

In case you wonder, why MongoDB casts undefined to null instead of just ignoring such properties - ignoreUndefined flag could solve this behaviour.

https://mongodb.github.io/node-mongodb-native/2.1/reference/connecting/connection-settings/