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MongoDB Tutorial

MongoDB: cursor.readPref() method

cursor.readPref

The cursor.readPref() method is used to specifiy a read preference to a cursor to control how the client directs queries to members of the replica set.

Syntax:

cursor.readPref(mode, tagSet)

Parameters:

Name Description Required /
Optional
Type
mode One of the following read preference modes: primary, primaryPreferred, secondary, secondaryPreferred, or nearest Required string
tagSet A tag set used to specify custom read preference modes. For details. Optional array

NOTE:You must apply readPref() to the cursor before retrieving any documents from the database.

Sample document in the restaurants collection:


{
  "address": {
     "building": "1007",
     "coord": [ -73.856077, 40.848447 ],
     "street": "Morris Park Ave",
     "zipcode": "10462"
  },
  "borough": "Bronx",
  "cuisine": "Bakery",
  "grades": [
     { "date": { "$date": 1393804800000 }, "grade": "A", "score": 2 },
     { "date": { "$date": 1378857600000 }, "grade": "A", "score": 6 },
     { "date": { "$date": 1358985600000 }, "grade": "A", "score": 10 },
     { "date": { "$date": 1322006400000 }, "grade": "A", "score": 9 },
     { "date": { "$date": 1299715200000 }, "grade": "B", "score": 14 }
  ],
  "name": "Morris Park Bake Shop",
  "restaurant_id": "30075445"
}
.....

Example: MongoDB: cursor.readPref() method

db.restaurants.find({"cuisine" : "American "}).readPref();

Output:

> db.restaurants.find({"cuisine" : "American "}).limit(2).readPref();
{ "_id" : ObjectId("55c3043ab165fa6355ec5c8c"), "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 }, { "date" : ISODate("2012-04-13T00:00:00Z"), "grade" : "A", "score" : 12 }, { "date" : ISODate("2011-10-12T00:00:00Z"), "grade" : "A", "score" : 12 } ], "name" : "Riviera Caterer", "restaurant_id" : "40356018"
 }
{ "_id" : ObjectId("55c3043ab165fa6355ec5c8e"), "address" : { "building" : "8825", "coord" : [ -73.8803827, 40.7643124 ], "street" : "Astoria Boulevard", "zipcode" : "11369" }, "borough" : "Queens", "cuisine" : "American ", "grades" : [ { "date" : ISODate("2014-11-15T00:00:00Z"), "grade" : "Z", "sco
re" : 38 }, { "date" : ISODate("2014-05-02T00:00:00Z"), "grade" : "A", "score" : 10 }, { "date" : ISODate("2013-03-02T00:00:00Z"), "grade" : "A", "score" : 7 }, { "date" : ISODate("2012-02-10T00:00:00Z"), "grade" : "A", "score" : 13 } ], "name" : "Brunos On The Boulevard", "restaurant_id" : "4035615
1" }

Retrieve the restaurants data from here



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