MongoDB: The Definitive Guide

Chapter 4. Querying

This chapter looks at querying in detail. The main areas covered are as follows:

• You can perform ad hoc queries on the database using the find or findOne functions and a query document.

• You can query for ranges, set inclusion, inequalities, and more by using $ conditionals.

• Some queries cannot be expressed as query documents, even using $ conditionals. For these types of complex queries, you can use a $where clause to harness the full expressive power of JavaScript.

• Queries return a database cursor, which lazily returns batches of documents as you need them.

• There are a lot of metaoperations you can perform on a cursor, including skipping a certain number of results, limiting the number of results returned, and sorting results.

Introduction to find

The find method is used to perform queries in MongoDB. Querying returns a subset of documents in a collection, from no documents at all to the entire collection. Which documents get returned is determined by the first argument to find, which is a document specifying the query to be performed.

An empty query document (i.e., {}) matches everything in the collection. If find isn’t given a query document, it defaults to {}. For example, the following:

> db.c.find()

returns everything in the collection c.

When we start adding key/value pairs to the query document, we begin restricting our search. This works in a straightforward way for most types. Integers match integers, booleans match booleans, and strings match strings. Querying ...