Rails + MongoID - Querying by attribute

Question

I have a model like this:

class Lesson
  include Mongoid::Document

  field :title, :type => String
  field :category, :type => String
  field :price, :type => Float
  field :description, :type => String
  field :user_id, :type => String


  validates_presence_of :title
  validates_presence_of :category
  validates_presence_of :price
  validates_presence_of :user_id

  attr_accessible :title, :category, :description, :price

end

And I am trying to query like this:

@lessons_by_user = Lesson.find_by_user_id current_user.id

And I am getting:

undefined method `find_by_user_id' for Lesson:Class

How can I query by a specific attribute in MongoID?

I know how to do it like this:

@lessons = Lesson.all(:conditions=>{:user_id=>current_user.id.to_s}) 

but I am wondering if there is a shortcut...

Answer 1

Mongoid doesn't have the ActiveRecord style auto-created finder methods, it only supports a limited set of predefined finder methods :

• Model.all
• Model.count
• Model.exists?
• Model.find
• Model.find_or_create_by
• Model.find_or_initialize_by
• Model.first
• Model.last

However, it does have a general purpose where method so you say this:

@lessons = Lesson.where(:user_id => current_user.id)

where is chainable as well (just like where in newer versions of ActiveRecord) so you can add more conditions or specify the ordering by chaining more criteria calls.

Answer 2

Since the version 3.0.0 of Mongoid, you can also do:

@lessons = Lesson.find_by(user_id: current_user.id)

Contrary to where , it will raise a Mongoid::Errors::DocumentNotFound exception if the request returns no result. It's the default behavior, but if you set the raise_not_found_error configuration option to false , it will just return nil in this case.

Source: http://mongoid.org/en/mongoid/docs/querying.html