• $returnKey
    • Note

Only return the index field or fields for the results of the query. If $returnKey is set to true and the query does not use an index to perform the read operation, the returned documents will not contain any fields. Use one of the following forms:

db.collection.find( { <query> } )._addSpecial( "$returnKey", true )
db.collection.find( { $query: { <query> }, $returnKey: true } )