This document discusses using MongoDB and a relational database together in applications. It suggests testing your data and access patterns to determine if MongoDB is a good fit for parts of your app. If so, utilities, logging, comments, and isolated subsystems are examples of good candidates for MongoDB. The document provides instructions for setting up MongoDB with Rails applications using MongoMapper and ActiveRecord together. It also describes some challenges like replacing migrations and tool upgrades over time.

1. Building Good Relations
Using MongoDB and a relational database in your apps

2. Hayes Davis
@hayesdavis
CheapTweet.com

3. If you’ve already got
an app

4. You probably already
have a database

5. It’s probably relational
http://www.mbari.org/ssds/ReferenceDocuments/RDB_ER.gif

6. It’s probably
gotten a little
bit big
http://www.neonlite.ca/archives/images/big%20cat.jpg

7. Or maybe it’s
gotten huge
http://www.fahad.com/pics/liger.jpg

8. And it’s probably
pretty complicated
http://www.mazes.org.uk/i/101005m.gif

9. Or maybe
really
complicated
http://tommilsom.com/wp-content/files/france/maze.jpg

10. Which makes it harder
to change
http://twitter.com/dacort/status/11023722831

11. But, you’re not
going to throw
all that out
http://blog.makezine.com/284163191_6f09179853_o.jpg

12. So let’s make NoSQL
and SQL live in harmony
http://curiousanimals.net/wp-content/uploads/2008/03/cat-and-dog-sleep.jpg

13. But first...

14. Do you even need to?

15. We all want to
play with shiny
new toys
http://actionfan.files.wordpress.com/2009/07/voltron_metallic-vinyl-edit.jpg

16. But sometimes
that’s not the
best plan
http://www.pwn3d.us/wp-content/uploads/2006/11/voltron_1.jpg

17. So, give yourself a test

18. #1
Is my data relational?

19. #1
Are my data relational?

20. #2
Do my data access
patterns lend themselves
to denormaliation?

21. #3
Do I expect a lot of
schema change?

22. #4
Can I drop ACID?
(Atomicity, Consistency, Isolation, Durability)

23. #5
Do I want to support a
new piece of
infrastructure?

24. If you answered yes to
any of these questions...

25. ... at least for parts of
your app...

26. ... then
MongoDB
might be
right for you
http://www.rankopedia.com/CandidatePix/25781.gif

27. Where to start...

28. Utilities and supporting
tools

29. Logging & analysis
http://sbadrinath.files.wordpress.com/2009/03/different26rqcu3.jpg

30. We built Parrot
http://filmfanatic.org/reviews/wp-content/uploads/2008/01/anfscd-parrot.png

31. Isolated
subsystems
http://themarkvolta.files.wordpress.com/2009/01/lost-map.jpg

32. Comments might be a
good choice
http://www.readwriteweb.com/archives/facebook_wants_to_be_your_one_true_login.php

33. Getting practical
(in Rails, at least)

34. Using MongoMapper
and ActiveRecord

35. Getting set up

36. config/mongodb.yml
development:
host: 127.0.0.1
test:
host: 127.0.0.1
production:
host: mongo-server.local

37. config/initializers/mongo.rb
# Read in the config info
mongo_cfg = YAML.load(
IO.read("#{RAILS_ROOT}/config/mongodb.yml")
)[RAILS_ENV]
# Create the connection from mongodb.yml
args = [mongo_cfg['host'],mongo_cfg['port']].compact
MongoMapper.connection =
Mongo::Connection.new(*conn_args)
# Pick the Mongo database
db_cfg =
Rails.configuration.database_configuration[RAILS_ENV]
MongoMapper.database =
mongo_cfg['database'] || db_cfg['database']

38. Mixing your MM with
your AR
http://i.treehugger.com/images/2007/10/24/frankenstein-jj-001.jpg

39. app/models/
preferences.rb
class Preferences
include MongoMapper::Document
key :user_id, Integer
end

40. apps/models/user.rb
class User < ActiveRecord::Base
after_save :save_prefs
def preferences
@preferences ||= Preferences.find_or_create_by_user_id(id)
end
private
def save_prefs
@preferences.save if @preferences
end
end

41. Usage
# Set any preferences you like
user = User.find_by_login('thatguy')
user.preferences[:foreground] = '#FF0000'
user.preferences[:foo] = 'bar'
user.save
# Later on, you can do this
user = User.find_by_login('thatguy')
user.preferences[:foreground] #returns '#FF0000'

42. Our general solution

43. active-expando
(very, very alpha)

44. Any attribute, any time
# Grab a user
user = User.find(1)
# Now set an attribute that didn't exist before
user.expandos.foreground = '#FF0000'
# foreground is saved in Mongo
user.save
# Find any users with a red foreground color
users = User.expando_all(
:conditions=>{:foreground=>'#FF0000'}
)

45. Skip expandos, use delegate
class Post < ActiveRecord::Base
expando_config do
delegate :tags
end
end
p = Post.find(1)
p.tags = ['foo','bar']
p.save
# Find any Post with the tag "foo"
Post.expando_all(:conditions=>{'tags'=>'foo'})

46. It’s a rails plugin

47. It’s on GitHub
(http://github.com/hayesdavis/active-expando)

48. Other issues

49. Where we’re going we
don’t need migrations
(or do we?)
http://www.gordtep.com/files/2009/08/bttf2.jpg

50. Flexibility is great
(until it bites you)

51. A general
word of
caution

52. MongoDB is a moving
target

53. So are the tools

54. There will be bugs
http://nitishkrishna.files.wordpress.com/2009/07/2007_there_will_be_blood_013.jpg

55. So, be ready to upgrade

56. Questions?