How to query MongoDB to test if an item exists?

Question

Does MongoDB offer a find or query method to test if an item exists based on any field value? We just want check existence, not return the full contents of the item.

Answer 1

Starting Mongo 2.6, count has a limit optional parameter, which makes it a viable alternative to find whether a document exists or not:

db.collection.count({}, { limit: 1 })
// returns 1 if exists and 0 otherwise

or with a filtering query:

db.collection.count({/* criteria */}, { limit: 1 })

Limiting the number of matching occurrences makes the collection scan stop whenever a match is found instead of going through the whole collection.

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Starting Mongo 4.0.3, since count() is considered deprecated we can use countDocuments instead:

db.collection.countDocuments({}, { limit: 1 })

or with a filtering query:

db.collection.countDocuments({/* criteria */}, { limit: 1 })