CODEWITHSUNDEEP
pythonmongodbpymongo
Imran
Imran

Reputation: 423

Mongo Query that always returns zero documents

It's possible to write a query that always returns all of the elements in a collection, to use pymongo as an example:

MongoClient()["database"]["collection"].find({})

However, due to the structure of my code, I would quite like to be able to construct a query that does the opposite, a query that will necessarily return zero elements in all situations:

MongoClient()["database"]["collection"].find(null_query)

How can I define null_query, such that this is correct?

Upvotes: 3

Views: 1376

Answers (1)

HaskellElephant
HaskellElephant

Reputation: 9891

You can ask for any field to be in an empty list. It seems reasonable to use the _id field for this:

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

If you want a shorter query you don't need to use the _id field at all:

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

Alternative if MongoDB version >= 3.4:

Arguably one can also ask if the _id field does not exists, which has been suggested by @Marco13:

db.collection.find({_id: {$exists: false}})

However, this assumes that all documents have the _id field, which is not necessarily true for MongoDB versions before 3.4 where a collection could be created with db.createCollection("mycol", {autoIndexID : false}) so all documents were not automatically given an _id field.

Upvotes: 2

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