Find documents where array has only identical elements

[torn...@gmail.com]

Hello,
let's assume I have the following partial documents:

{StringArray: ["A","B","C"]}
{StringArray: ["A","A","A"]}
{StringArray: ["B","B","B"]}

Now i want to find all the documents where StringArray only contains "A" elements. Let's name this the ALL operation.

My current way is:
db.TestCollection.find({ StringArray: { $elemMatch: {$eq: "A"}, $not: {$elemMatch: {$ne: "A"}}}})

which makes sure that i only hit the documents where StringArray contains 'A'  AND does not contain anything which is not 'A'. (Without the first part it would also match empty arrays).

Now this works for the simple cases, for more complicated elemMatch expressions (regex, other logical operators) this is still possible, but gets complicated to build the appropriate negated expression.

It gets really complicated,  if I have multiple nested ALL operations (e.g. find all document where all the embedded documents all contain the string array above)

Has anyone a better idea on how to do this ALL operation without the need to manually build the negated expression?

Thanks,
Torni

[Asya Kamsky]

How about using aggregation framework? Multiple possibilities exist...

db.collection.aggregate({$project:{numUniqueStringArray:{$size:
{$setDifference:["$StringArray", [] ]}}}}, {$match:{
numUniqueStringArray : 1 } } )

That works to find all documents where StringArray contains one or
more of any single letter. If you specifically want "A" you can use [
"A" ] as second argument to $setDifference and then match results that
are 0 only.

Asya

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[torn...@gmail.com]

Thanks for the example with the aggregation framework. I'll try that.