Slides for Presentation given at MongoLA on January 13, 2011 at Media Temple.

1. MYSQL TO
MONGO (FTW)
Thinking Differently About Schema Design

2. @AJSHARP
Alex Sharp
Lead Developer, OptimisCorp
alexjsharp.com
github.com/ajsharp

3. CVBEAST
Mongo has many cool
features such as schema-
free, aggregation w
map/reduce and many
others
Side project
interested more in
domain modeling more
than performance and
scaling benefits of
Mongo
EMBEDDED OBJECTS

4. CVBEAST
App to represent people’s curriculum vitae, but not in an
academic sense

5. CVBEAST
Focus on “micro” experiences not suitable for a typical CV
(definitely not suitable for a resumé)

6. CVBEAST
Simple object model, centered around highlighting attributes
of a person

7. CVBEAST
Started out building with MySQL, b/c it’s familiar

8. OBJECT MODEL
Person
has a
CV
has many
Experience
has many
... ... ...

9. OBJECT MODEL
Person
has a
CV
links, tags, and has many
other arbitrary
properties Experience
has many
... ... ...

10. RELATIONAL SCHEMA
people
- id
- name
- ...
cvs
- id
- person_id
- ...
experiences
- id
- cv_id
- ...

11. RELATIONAL SCHEMA
Lots of pointless JOINs

12. RELATIONAL SCHEMA
people
- id
- name
- ...
cvs
- id
- person_id
- ...
experiences
- id
- cv_id
- ...

13. RELATIONAL SCHEMA
people
- id
- name
- ...
cvs tags
- id
- id - name
- person_id - ...
- ...
links
experiences - id
- name
- id - ...
- cv_id
- ...

14. not b/c of premature
optimization
this bothers me

15. It misrepresents the object model
at the storage layer

16. It also became difficult to work with
despite my familiarity with MySQL

17. It makes sense to store object relationships in
first-class entities/documents/tables

18. But true object properties should be
stored as properties

19. Not relationships.

20. Especially when properties have no practical
meaning without the parent

21. CAVEAT
This is not always the case.
;-)
if you need to:
* store LOTS of embedded objects (i.e. papermill)

22. EXAMPLE
Experience Links

23. EXPERIENCE.LINKS = [...]
Dead-simple collection of links attached to
an experience

25. RUBY MODEL CODE
class Person
include Mongoid::Document
field :keywords, :type => Array
index :keywords
index 'cv.experiences.tags'
end

26. class Experience
include Mongoid::Document
embeds_many :links
end
class Link
include Mongoid::Document
field :url
field :title
embedded_in :experience, :inverse_of => :links
end

27. class Experience
include Mongoid::Document
embeds_many :links
end Both models are embedded in the
person collection
class Link
include Mongoid::Document
field :url
field :title
embedded_in :experience, :inverse_of => :links
end

28. EXPERIENCE.LINKS = [...]
A link is only relevant inside the
context of an experience object.

29. EXPERIENCE.LINKS = [...]
In other words, it is a property,
not a relationship.

30. EXPERIENCE.LINKS = [...]
Mongo brings the storage layer closer to
the object model

31. EXPERIENCE.LINKS = [...]
{
"title": "Presented at MongoLA",
"links": [
{ "title": "Event Site",
"url": "http://www.10gen.com/conferences/mongola2011"},
{ "title": "Slides",
"url": "http://alexjsharp.com/posts/mongola-2010-slides"}
]
}

32. EXAMPLE
Embedded Search

34. MYSQL
An exercise in masochism

35. The difficulty in MySQL came in working with
properties of a person

36. These properties are represented as tables

37. And tables must be joined

38. SIMPLE SEARCH QUERY
select * from people
inner join cvs on cvs.person_id = people.id
inner join experiences on experiences.cv_id = cvs.id
inner join tags on tags.experience_id = experiences.id
where tags.name = 'ruby';

39. INDEXES NEEDED
- tags.experience_id
- tags.name
- experiences.cv_id
- cvs.person_id

40. Seems extremely unnecessary for such a
simple object model

41. MONGO
An exercise in ... not masochism

42. Embedded objects make this query easy.

43. MONGO DOCUMENT “SCHEMA”
{"name": "Alex Sharp",
"cv": {
"experiences": [
{ "title": "spoke at MongoLA",
"date" : "Thu Jan 13 2011",
"tags" : ["mongodb", "speaking"]
},
{"title": "..."}
]
}
}

44. MONGO DOCUMENT “SCHEMA”
{"name": "Alex Sharp",
"cv": {
"experiences": [
{ "title": "spoke at MongoLA",
"date" : "Thu Jan 13 2011",
"tags" : ["mongodb", "speaking"]
},
{"title": "..."}
]
}
}
we want to search for these

45. RUBY MODEL CODE
class Person
include Mongoid::Document
field :keywords, :type => Array
index :keywords
index 'cv.experiences.tags'
end

46. RUBY MODEL CODE
class Cv
include Mongoid::Document
embeds_many :experiences
end
class Experience
include Mongoid::Document
field :tags, :type => Array, :default => []
embedded_in :cv, :inverse_of => :experiences
embeds_many :links
after_save lambda { |exp| exp.cv.person.update_keywords }
end

47. RUBY MODEL CODE
class Cv
include Mongoid::Document
simple property
embeds_many :experiences
end (not relationship)
class Experience
include Mongoid::Document
field :tags, :type => Array, :default => []
embedded_in :cv, :inverse_of => :experiences
embeds_many :links
after_save lambda { |exp| exp.cv.person.update_keywords }
end

48. RUBY SEARCH CODE
class Person
# i.e. db.people.find({"cv.experiences.tags": "ruby"})
def self.search_by_tag(term)
collection.find('cv.experiences.tags' => 'ruby')
end
end

49. COMPARISON
select * from people
inner join cvs on cvs.person_id = people.id
inner join experiences on experiences.cv_id = cvs.id
inner join tags on tags.experience_id = experiences.id
where tags.name = 'ruby';
vs
db.people.find('cv.experiences.tags' => 'ruby')

50. WINS: IMPEDENCE MIS-MATCH
Object persistence format is closer to
application usage format

51. WINS: F(X)-ALITY
Plus, we don’t lose any functionality.

52. WINS: FAMILIARITY
Indexing principles are extremely familiar to
relational database users.

53. WINS: FAMILIARITY
Only simpler ;)

54. WINS: SIMPLICITY
Query syntax is simple.

55. WINS: SIMPLICITY
Dot notation > JOIN semantics filth

56. SUMMARY
Mongo is perfectly suited for
an app like CVBeast.

57. SUMMARY
Where a relational DB forces storing
object properties as relationships

58. SUMMARY
Mongo narrows this mismatch considerably

59. SUMMARY
A CV is a document...

60. SUMMARY
So a document-oriented datastore
seems appropriate.

61. SUMMARY
Many object models have this
tree-like structure.

62. SUMMARY
Be willing to step outside of your comfort zone

63. SUMMARY
And use Mongo when it’s practical.