Why mongodb finds this document ($ne: null for array)

Question

Data:

db.inventory.insertMany([
       { _id: 1, item: null },
       { _id: 2 },
       { _id: 3, item: 3 },
       { _id: 4 items: [1, 2, 3] },
       { _id: 5, items: [] }
    ])

Query 1:

db.inventory.find({ 'item': {$ne: null} })

Result 1:

{ _id: 3, item: 3 }

Query 2:

db.inventory.find({ 'items.0': {$ne: null} })

Result 2:

{ _id: 3, items: [1, 2, 3] },
{ _id: 4, items: [] }

Why mongoDB finds this document: { _id: 4, items: [] }? Is this a bug?

D. SM · Accepted Answer · 2020-05-20 19:42:12Z

2

Use $exists: true instead of $ne: null.

$ne docs explain:

$ne selects the documents where the value of the field is not equal to the specified value. This includes documents that do not contain the field.

answered May 20, 2020 at 19:42

D. SM

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3 Comments

Sergkash Over a year ago

docs.mongodb.com/manual/tutorial/query-for-null-fields In this example, the query db.inventory.find ({item: null}) displays a document without item. Why does this not work with arrays?

D. SM Over a year ago

I don't know, you can ask on developer.mongodb.com/community/forums.

Sergkash Over a year ago

Thanks for the link. developer.mongodb.com/community/forums/t/…

Nafeo Alam · Accepted Answer · 2020-05-21 05:26:22Z

0

null itself a datatype in mongodb. this is why at the 0 index of items it is searching for a null value and not getting it. if you add null like items: [null] and run the query again you will not see _id: 4
This is not a bug. this is how Mongo works.
In your first result, the key item in the object itself does not exist. To satisfy 'item': {$ne: null} the key itself has to be present in the object. This is how javascript works as well. Check the JS Code.
It also works with arrays. try: {'items.0': null} and {'items.0': {$ne: null}}, and you should see the difference.

edited May 21, 2020 at 5:26

answered May 20, 2020 at 20:11

Nafeo Alam

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2 Comments

Sergkash Over a year ago

But why the first result is missing document { _id: 2 }.

Sergkash Over a year ago

docs.mongodb.com/manual/tutorial/query-for-null-fields In this example, the query db.inventory.find ({item: null}) displays a document without item. Why does this not work with arrays?

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