This document summarizes MongoMapper, an ORM for MongoDB. MongoMapper allows modeling of objects and relationships like ActiveRecord but for MongoDB. It supports common data types, custom types, validations, associations like many-to-many and polymorphic relationships, querying, and modifiers like increment. Some gotchas include lack of transactions, uniqueness being per class not collection, and needing type hints for forms. Reflection on the switch notes acceptance tests were valuable while unit tests were painful to migrate. The document recommends decoupling behavior from persistence.

1. MongoMapper
Kerry Buckley
IPRUG Lightning talk, 2 August 2011

2. What is it?

3. What is it?

4. Why we switched

5. Why we switched
• Modelling complex object hierarchy
• Painfully-complex polymorphic associations
• (and about to get worse)
• Data seemed to suit a document DB

6. Document
class User
include MongoMapper::Document
key :username, String
key :real_name, String
key :date_of_birth, Date
end

7. Supported types
• Array • Object • Date
• Float • String • ObjectId
• Hash • Time • Set
• Integer • Binary
• NilClass • Boolean

8. Custom types
module MongoMapper::BigDecimal
def to_mongo value
value.to_s
end
def from_mongo value
value.present? ? new(value.to_s) : nil
end
end
BigDecimal.send :extend, MongoMapper::BigDecimal
class Person
include MongoMapper::Document
key :height, BigDecimal
end

9. Database migrations

10. Database migrations
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11. Time and user stamps
class Article
include MongoMapper::Document
key :title, String
key :body, String
timestamps!
userstamps!
end
class User
include MongoMapper::Document
key :name, String
end

12. Simple associations
class Owner
include MongoMapper::Document
many :pets
end
class Pet
include MongoMapper::Document
belongs_to :owner
one :bed
end
class Bed
include MongoMapper::Document
belongs_to :pet
end

13. Embedded document
class Order
include MongoMapper::Document
many :line_items
end
class LineItem
include MongoMapper::EmbeddedDocument
key :name, String
key :quantity, Integer
end

14. Many-to-many
class Book
include MongoMapper::Document
key :title
key :author_ids, Array
many :authors, :in => :author_ids
end
class Author
include MongoMapper::Document
key :name
end

15. Basic polymorphism
class BlogPost
include MongoMapper::Document
key :body
many :comments, :as => :commentable
end
class HomePage
include MongoMapper::Document
key :content
many :comments, :as => :commentable
end
class Comment
include MongoMapper::Document
key :text
belongs_to :commentable, :polymorphic => true

16. SCI polymorphism
class Commentable
include MongoMapper::Document
many :comments
end
class BlogPost < Commentable
key :body
end
class HomePage < Commentable
key :content
end
class Comment
include MongoMapper::Document
key :text
belongs_to :commentable
end

17. Reverse polymorphism
class Site
include MongoMapper::Document
many :pages
end
class Page
include MongoMapper::Document
belongs_to :site
end
class HomePage < Page
key :content
end
class BlogPost < Page
key :body

18. M2M polymorphism
class Book
include MongoMapper::Document
key :author_ids, Array
many :authors, :in => :author_ids
end
class Novel < Book
end
class Textbook < Book
end
class Author
include MongoMapper::Document
end
class Editor < Author
end

19. Validations
class Member
include MongoMapper::Document
key :name, String
key :age, Integer
key :sex, String
validates_uniqueness_of :name
validates_presence_of :name
validates_numericality_of :age
validates_inclusion_of :sex, :in => %w{male female}
validate :custom_validation
def custom_validation
...
end
end

20. Validation shorthand
class Member
include MongoMapper::Document
key :name, String, :required => true, :unique => true
key :age, Integer, :numeric => true
key :sex, String, :in => %w{male female}
validate :custom_validation
def custom_validation
...
end
end

21. Querying
class User
include MongoMapper::Document
key :first_name, String
key :last_name, String
scope :smiths, where(:last_name => "Smith")
end
User.find "4e0c70aea7a6c32ab5000003"
User.find_by_first_name "Kerry"
User.find_by_first_name! "Kerry"
User.find_by_first_name_and_last_name "Kerry", "Buckley"
User.find_all_by_last_name "Buckley"
User.smiths.find_by_first_name "John"

22. Modifiers
class Page
include MongoMapper::Document
key :title, String
key :hit_count, Integer, :default => 0
key :tags, Array
end
@page.increment :hit_count => 1
Page.increment {:title => "Home"}, :hit_count => 1
Page.set {:title => "Home"}, :title => "New Home"
Page.unset {:title => "Home"}, :title
Page.push {:title => "Home"}, :tags => "foo"

23. Gotchas

24. Gotchas
• No transactions
• find versus find!
• Writes are fire-and-forget by default
• Uniqueness is per class, not per collection
• Formtastic needs hints of field type

25. Reflections

26. Reflections
• Longish but fairly painless switch
• Acceptance/integration tests invaluable
• Specs/unit tests a pain to migrate
• Decouple behaviour from persistence!

27. Fin.