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:

:count :length :size	Author.count Author.where(name: 'Charles').length Author.where(name: 'Charles').size	This finds the number of documents in the collection.
:distinct :pluck	Author.distinct(:name) Author...