MongoDB find Documents where all array elements equal some value

Question

Collection:

db.test.find()
{
  { "_id" : ObjectId(...), "arr" : [ "Today", "is", null ] }
  { "_id" : ObjectId(...), "arr" : [ null, null, null ] }
}

I'm trying to find all documents where all of arr equals some value. In this example, I would want the document containing arr : [null, null, null] when given null.

Find documents where ALL elements of an array have a specific value

This solution is close to what I want; however, my array data do not have keys for an $elemMatch to reference. Is there a way to accomplish this query without being unnecessarily costly or restructuring my data?

Thanks!

There is very similar question here as well.stackoverflow.com/questions/23595023/… — s7vr
– s7vr, Commented Apr 24, 2017 at 18:45

s7vr · Accepted Answer · 2017-04-24 18:50:04Z

3

You can use $elemMatch query operator. It just needs a query.

 db.test.find( { arr:  { $not: { $elemMatch: { $ne: null } } } } )

"$elemMatch" + "$ne"

This part includes all the documents where arr array don't have at least one null value.

These are all the documents which has at least one not null value.

$not

This part will keep the all the documents which are not in "$elemMatch" + "$ne".

These are all the documents that has its all of values as null.

Please accommodate edge cases where field doesn't exist to make sure things work as expected.

edited Apr 24, 2017 at 18:50

answered Apr 24, 2017 at 18:37

s7vr

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1 Comment

Chris Over a year ago

This appears to work, with the addition of an exists and size check on arr. Thanks!

helmy · Accepted Answer · 2017-04-24 18:23:29Z

0

You can use the the $all operator, but the semantics might not be exactly what you want.

db.test.find({ arr: { $all: [null] } })

That would return both of your test documents.

If you really want to query for an array with 3 null values, you can just query for the literal array:

db.test.find({arr:[null,null,null]})

answered Apr 24, 2017 at 18:23

helmy

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3 Comments

Chris Over a year ago

Correct, the $all operator seems to return if any value is null (including arr itself). Unfortunately, it is unknown how many values an array will have, so I will be unable to construct a literal array for comparison.

helmy Over a year ago

It seems like an odd requirement. Perhaps there's another approach to solving. Can you share more details regarding your use case and examples?

Chris Over a year ago

Simply put, I need to be able to filter documents who have homogeneous data in an array. For example, say driver1 and driver2 have the following entries in db.drivers: {_id: "driver1" , record: ["win", "win", "win"]}, {_id: "driver2", record: ["win"]}. Given this data, I would want to query db.drivers for all drivers with a perfect record, which would return both in this case.

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