cursor.returnKey()¶

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Definition
Behavior
Example

Definition¶

cursor.returnKey()¶

3.2 新版功能.

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

The cursor.returnKey() has the following form:

cursor.returnKey()

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

Behavior¶

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

Example¶

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" }
...

← cursor.readPref() cursor.showRecordId() →