Find mongo docs with empty object in array

Question

I have a Mongo collection with an array property, and the array has "empty" objects in it. What query can I use to target these non-empty arrays that have only {}?

I've tried combinations of $exists and $where to no avail.

Examples of data I need to target:

"arrayProperty" : [ {} ]

and

"arrayProperty" : [ {}, {} ]

EDIT:

Here's an example of the schema:

{
    "_id" : ObjectId("53b1ca583d597ce7cbd54646"),
    "arrayProperty" : [ 
        {
            "serialNumber" : "abc123",
            "rfid" : "xyz098",
            "size" : 95,
            "points" : 50,
            "frequency" : "Every day",
            "dateAssigned" : ISODate("2011-02-10T15:27:39.000Z")
        }
    ]
}

Are you looking for docs where ANY property contains an empty document? How about if it is not at root level such as prop1: {prop2: {}} — kwolfe
– kwolfe, Commented Jul 14, 2014 at 20:23
Are you looking to find docs where arrayProperty contains only {} objects? For example, should arrayProperty: [ {}, {a: 1} ] match? — JohnnyHK
– JohnnyHK, Commented Jul 14, 2014 at 20:43
I'm looking for root properties such as doc.arrayProperty: [{}]. There are also some like doc.arrayProperty: [{}, {}] — Erik Olson
– Erik Olson, Commented Jul 15, 2014 at 1:12

JohnnyHK · Accepted Answer · 2014-07-15 18:24:09Z

7

If you're looking to find docs where arrayProperty contains at least one {} then it's just:

db.collection.find({ arrayProperty: {} })

If you're looking to find docs where arrayProperty only contains either one or two {}:

db.collection.find({ arrayProperty: {$in: [ [{}], [{}, {}] ] } })

answered Jul 15, 2014 at 18:24

JohnnyHK

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1 Comment

Erik Olson Over a year ago

I set up a dedicated test collection, and this worked!. I also tested @Trust's solution on the test collection, and that worked too. I'm going to try to flag both answers as correct...hopefully I can. Thanks.

u3l · Accepted Answer · 2014-07-14 20:25:16Z

2

The following may/may-not work...

db.collection.aggregate([
{$unwind: {"$arrayProperty"}},
{$match: {arrayProperty: {} }},
{$group: {_id: "$_id"} }
])

Basically what I'm trying to do is unwind arrayProperty, pull out all documents with an arrayProperty as {}, and group all of the unwinded documents back that have the same _id (or you can replace that with whatever unique key field you use).

Again, I don't know for sure if this will work so try it out and let me know.

answered Jul 14, 2014 at 20:25

u3l

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7 Comments

Erik Olson Over a year ago

Didn't seem to work. I had to remove the { } around "$arrayProperty". Even with that I got.. { "result" : [], "ok" : 1 }

u3l Over a year ago

Could you perhaps include the schema of a typical document in your question? In any case, you might want to try experimenting with aggregation and see if you can come up with anything.

Erik Olson Over a year ago

I updated the question to include an example of the schema. Thanks.

u3l Over a year ago

Hmmm... okay I have to say I'm pretty stumped (sorry). Are you using a driver (eg: the Python driver or the Java driver)? If so, you could potentially just loop through arrayProperty from the driver itself and check if each element is an empty document. Unfortunately I'm not sure how else to do it... perhaps JohnnyHK's answer may work but that assumes that you know the maximum amount of empty docs arrayProperty could hold.

Erik Olson Over a year ago

I set up a dedicated test collection, and this worked!. I also tested @JohnnyHK's solution on the test collection, and that worked too. I'm going to try to flag both answers as correct...hopefully I can. Thanks.

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