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cursor.returnKey()

cursor.returnKey()
Important
mongo Shell Method

This page documents the mongo shell method, and does not refer to the MongoDB Node.js driver (or any other driver) method. For corresponding MongoDB driver API, refer to your specific MongoDB driver documentation instead.

Tip

Starting in MongoDB 4.4, $meta supports the keyword "indexKey" to return index key metadata if an index is used. The use of { $meta: "indexKey" } is preferred over cursor.returnKey().

Modifies the cursor to return index keys rather than the documents.

The cursor.returnKey() has the following form:

cursor.returnKey()
Returns:The cursor that returnKey() is attached to with a modified result set. This allows for additional cursor modifiers to be chained.

If the query does not use an index to perform the read operation, the cursor returns empty documents.

The restaurants collection contains documents with the following schema:

{
"_id" : ObjectId("564f3a35b385149fc7e3fab9"),
"address" : {
"building" : "2780",
"coord" : [
-73.98241999999999,
40.579505
],
"street" : "Stillwell Avenue",
"zipcode" : "11224"
},
"borough" : "Brooklyn",
"cuisine" : "American ",
"grades" : [
{
"date" : ISODate("2014-06-10T00:00:00Z"),
"grade" : "A",
"score" : 5
},
{
"date" : ISODate("2013-06-05T00:00:00Z"),
"grade" : "A",
"score" : 7
}
],
"name" : "Riviera Caterer",
"restaurant_id" : "40356018"
}

The collection has two indexes in addition to the default _id index:

{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "guidebook.restaurant"
},
{
"v" : 1,
"key" : {
"cuisine" : 1
},
"name" : "cuisine_1",
"ns" : "guidebook.restaurant"
},
{
"v" : 1,
"key" : {
"_fts" : "text",
"_ftsx" : 1
},
"name" : "name_text",
"ns" : "guidebook.restaurant",
"weights" : {
"name" : 1
},
"default_language" : "english",
"language_override" : "language",
"textIndexVersion" : 3
}

The following code uses the cursor.returnKey() method to return only the indexed fields used for executing the query:

var csr = db.restaurant.find( { "cuisine" : "Japanese" } )
csr.returnKey()

This returns the following:

{ "cuisine" : "Japanese" }
{ "cuisine" : "Japanese" }
{ "cuisine" : "Japanese" }
{ "cuisine" : "Japanese" }
...
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