Validations

Mongoid includes ActiveModel::Validations, so we have access to all the ActiveModel validations that are available. In a nutshell, validations ensure the sanity of the data being stored. To quote a quick example:

class Author
  include Mongoid::Document

  field :name, type: String

  validates :name, presence: true
end   

This ensures that no author object will be created without a name. Similarly, there are options such as, :uniqueness, :acceptance, :associated, :confirmation, which are from ActiveModel::Validations, that allow checks for uniqueness of attributes, presence of associated models, and so on. The common options used along with these validations are :if, :allow_nil, and :on. You can find a lot more details about validations at http://api.rubyonrails.org/classes/ActiveModel/Validations/ClassMethods.html.