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Deprecated in the mongo Shell since v3.2
Starting in v3.2, the $returnKey operator is deprecated in the mongo shell. In the mongo shell, use cursor.returnKey() instead.

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 } )
←   $query $showDiskLoc  →