Finding Distinct Field Values

A very useful query against a MongoDB collection is to get a list of the distinct values for a single field in a set of documents. Distinct in this case means that even though there are thousands of documents, you only want to know the unique values that exist.

The distinct() method on Collection objects allows you to find a list of distinct values for a specific field. The syntax for the distinct() method is shown below:

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distinct(key,[query],[options],callback)

The key parameter is the string value of the fieldname you want to get values for. You can specify subdocuments by using the dot syntax—for example, stats.count. The query parameter is an object with the standard query object options listed in Table 15.1. The options parameter is an options object that allows you to define the readPreference option defined in Table 15.2. The callback function accepts an error as the first parameter and then a results parameter, which is an array of distinct values for the field specified in the key parameter.

Listing 15.7 shows how to find the distinct values in the words collection. Notice that the query in line 14 limits the words to those starting with u. Notice that line 18 uses dot syntax to access the stats.vowels field. Figure 15.7 shows the output of Listing 15.7.

Listing 15.7 doc_distinct.js: Finding distinct field values in a specific set of documents in a MongoDB collection

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01 var MongoClient = require('mongodb').MongoClient;
02 MongoClient.connect("mongodb://localhost/", function(err, db) {
03   var myDB = db.db("words");
04   myDB.collection("word_stats", distinctValues);
05   setTimeout(function(){
06     db.close();
07   }, 3000);
08 });
09 function distinctValues(err, words){
10   words.distinct('size', function(err, values){
11     console.log(" Sizes of words: ");
12     console.log(values);
13   });
14   words.distinct('first', {last:'u'}, function(err, values){
15     console.log(" First letters of words ending in u: ");
16     console.log(values);
17   });
18   words.distinct('stats.vowels', function(err, values){
19     console.log(" Numbers of vowels in words: ");
20     console.log(values);
21   });
22 }

Figure 15.7 Finding distinct field values in a specific set of documents in a MongoDB collection.