You can query CouchDB for documents by their ID, but of course, most of the time, you'll want to issue more complex queries, such as matching a field in a record against a particular value. CouchDB lets you define views of your data that consist of an arbitrary key in a collection of objects and then the objects derived from the view. When you specify a view, you're specifying two JavaScript functions: a map function that maps keys to items in your collection, and then an optional reduce function that iterates over the keys and values to create a final collection. In this recipe, we'll use the map function of a view to create an index of records by a single field.

db.save('_design/stations', {
  views: {
    byCall: {
      map: function(doc) {
        if (doc.call) {
          emit(doc.call, doc);
        }
      }
    }
  }
});