Book Image

Learning Mongoid

By : Gautam Rege
Book Image

Learning Mongoid

By: Gautam Rege

Overview of this book

Mongoid helps you to leverage the power of schema-less and efficient document-based design, dynamic queries, and atomic modifier operations. Mongoid eases the work of Ruby developers while they are working on complex frameworks. Starting with why and how you should use Mongoid, this book covers the various components of Mongoid. It then delves deeper into the detail of queries and relations, and you will learn some tips and tricks on improving performance. With this book, you will be able to build robust and large-scale web applications with Mongoid and Rails. Starting with the basics, this book introduces you to components such as moped and origin, and how information is managed, learn about the various datatypes, embedded documents, arrays, and hashes. You will learn how a document is stored and manipulated with callbacks, validations, and even atomic updates. This book will then show you the querying mechanism in detail, right from simple to complex queries, and even explains eager loading, lazy evaluation, and chaining of queries. Finally, this book will explain the importance of performance tuning and how to use the right indexes. It also explains MapReduce and the Aggregation Framework.
Table of Contents (14 chapters)
Learning Mongoid
About the Author
About the Reviewers

Mongoid criteria

The Mongoid::Criteria module is the core object for querying MongoDB. This includes the entire Origin DSL for querying along with other goodies. Every time a query is fired, a criterion is created. For example, in the following query, we do not get back a result but a criterion object:

irb> Book.exists(awards: true)
 => #<Mongoid::Criteria
  selector: {"awards"=>{"$exists"=>true}}
  options:  {}
  class:    Book
  embedded: false>

This criterion can now be chained with other criteria, just like ActiveRelation. This helps us fire a single query to the database only when all the criteria are fully resolved:

> Book.exists(awards: true).count
 => 20

In addition to the methods provided in Origin, some helpful criteria are mentioned as follows:




Author.where(name: 'Charles').length
Author.where(name: 'Charles').size

This finds the number of documents in the collection.