Limiting Fields Returned in Objects

Another extremely effective method of limiting the resulting data when retrieving documents is to limit which fields are returned. Documents may have a lot of different fields that are useful in some circumstance but not in others. You should consider which fields should be included when retrieving documents from the MongoDB server and only request the ones that are necessary.

To limit the fields returned from a server, use the fields option of the options object. The fields option in an object allows you to either include or exclude fields by setting the value of the document field to 0 for exclude or 1 for include. You cannot mix inclusions and exclusions in the same expression.

For example, to exclude the fields stats, value, and comments when returning a document, you use the following fields option:

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{fields:{stats:0, value:0, comments:0}}

Often it is easier to just include a few fields. For example, if you want to include only the name and value fields of documents, you use:

{fields:{name:1, value:1}}

Listing 15.4 shows how to use the fields option to reduce the amount of data returned from the server by excluding fields or specifying specific fields to include. Figure 15.4 shows the output of Listing 15.4.

Listing 15.4 doc_fields.js: Limiting the fields returned with a set of documents

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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", limitFields);
05   setTimeout(function(){
06     db.close();
07   }, 3000);
08 });
09 function limitFields(err, words){
10   words.findOne({word:'the'}, {fields:{charsets:0}},
11                 function(err, item){
12     console.log("Excluding fields object: ");
13     console.log(JSON.stringify(item, null, 2));
14   });
15   words.findOne({word:'the'}, {fields:{word:1,size:1,stats:1}},
16                 function(err, item){
17     console.log("Including fields object: ");
18     console.log(JSON.stringify(item, null, 2));
19   });
20 }

Figure 15.4 Limiting the fields returned in documents by using the fields option.