MapReduce Views
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MapReduce Views

You can use MapReduce views to create queryable secondary indexes in Couchbase Server.

The normal CRUD methods allow you to look up a document by its ID. A MapReduce (view query) allows you to look up one or more documents based on various criteria. MapReduce views are comprised of a map function that is executed once per document (this is done incrementally, so this is not run each time you query the view) and an optional reduce function that performs aggregation on the results of the map function. The map and reduce functions are stored on the server and written in JavaScript.

MapReduce queries can be further customized during query time to allow only a subset (or range) of the data to be returned.

Tip: See the Incremental MapReduce Views and Querying Data with Views sections of the general documentation to learn more about views and their architecture.

The following example is the definition of a by_name view in a "beer" design document. This view checks whether a document is a beer and has a name. If it does, it emits the beer's name into the index. This view allows beers to be queried for by name. For example, it's now possible to ask the question "What beers start with A?"

function (doc, meta) {
    if (doc.type && doc.type == "beer" && doc.name) {
        emit(doc.name, null);
    }
}

A Spatial View can instead be queried with a range or bounding box. For example, let's imagine we have stored landmarks with coordinates for their home city (eg. Paris, Vienna, Berlin and New York) under geo, and each city's coordinates is represented as two attributes, lon and lat. The following spatial view map function could be used to find landmarks within Europe, as a "by_location" view in a "spatial" design document:

function (doc, meta) {
    if (doc.type && doc.type == "landmark" && doc.geo) {
        emit([doc.geo.lon, doc.geo.lat], null);
    }
}

Querying Views through the Node.js SDK

Querying a view through the Node.js client is performed through the ViewQuery class that is available as a top-level object.

var couchbase = require('couchbase');
var ViewQuery = couchbase.ViewQuery;

var query = ViewQuery.from('beer', 'by_name');

A ViewQuery object enables you to query the view and specify various available options for the query. Once you have the query ready for execution, pass it to the query method of the your Bucket instance:

var myBucket = myCluster.openBucket();
myBucket.query(query, function(err, results) {
  for(i in results)
    console.log(results[i]);
});

You can modify your view results by specifying various options before executing the query method. Here is an example that skips the first six items and limits the results to three items:

var query = ViewQuery.from('beer', 'by_name').skip(6).limit(3);
myBucket.query(query, function(err, results) {
  for(i in results)
    console.log(results[i]);
});

If you are interested in performing a geospatial view, simply use SpatialQuery as your root rather than ViewQuery.

var SpatialQuery = couchbase.SpatialQuery;

var query = SpatialQuery.from('spatial', 'by_location').limit(10);
myBucket.query(query, function(err, results) {
  for(i in results)
    console.log(results[i]);
});