This document discusses ROM (Ruby Object Mapper), a library for building data access layers in Ruby. It summarizes ROM's key concepts including relations for reading data, commands for writing data, and repositories for accessing application data. It also highlights ROM's features like no global state, support for multiple databases and adapters, and avoidance of N+1 queries through relation composition.

2. Me
Nikita Shilnikov
• github.com/flash-gordon
• Whatever developer
• dry-rb and rom-rb core team member

3. ROM
disinterring the stewardess

5. Ruby Object Mapper

6. ROM vs ORM

7. ROM vs AR

8. ROM vs AR
user = Users.create(name: 'John Doe')
user.age = 30
user.save

9. ROM vs AR
builder = SqlBuilder.new <<-SQL
UPDATE topic_users tu
SET notification_level =
CASE WHEN should_track THEN :tracking
WHEN should_watch THEN :watching
ELSE notification_level
END,
notifications_reason_id =
CASE WHEN should_track THEN null
WHEN should_watch THEN :auto_watch_category
ELSE notifications_reason_id
END
FROM (
SELECT tu1.topic_id,
tu1.user_id,
CASE WHEN
cu.user_id IS NULL AND tu1.notification_level = :watching AND tu1.notifications_reason_id = :auto_watch_category THEN true
ELSE false
END should_track,
CASE WHEN
cu.user_id IS NOT NULL AND tu1.notification_level in (:regular, :tracking) THEN true
ELSE false
END should_watch
FROM topic_users tu1
JOIN topics t ON t.id = tu1.topic_id
LEFT JOIN category_users cu ON cu.category_id = t.category_id AND cu.user_id = tu1.user_id AND cu.notification_level = :watching
/*where2*/
) as X
/*where*/
SQL

10. ROM vs AR
--builder = SqlBuilder.new <<-SQL
UPDATE topic_users tu
SET notification_level =
CASE WHEN should_track THEN :tracking
WHEN should_watch THEN :watching
ELSE notification_level
END,
notifications_reason_id =
CASE WHEN should_track THEN null
WHEN should_watch THEN :auto_watch_category
ELSE notifications_reason_id
END
FROM (
SELECT tu1.topic_id,
tu1.user_id,
CASE WHEN
cu.user_id IS NULL AND tu1.notification_level = :watching AND tu1.notifications_reason_id = :auto_watch_category THEN true
ELSE false
END should_track,
CASE WHEN
cu.user_id IS NOT NULL AND tu1.notification_level in (:regular, :tracking) THEN true
ELSE false
END should_watch
FROM topic_users tu1
JOIN topics t ON t.id = tu1.topic_id
LEFT JOIN category_users cu ON cu.category_id = t.category_id AND cu.user_id = tu1.user_id AND cu.notification_level = :watching
/*where2*/
) as X
/*where*/
SQL

13. Ac#veRecord is not your Domain Layer

14. Code is not data

15. Data is $$$

16. Code is !

17. Code is live
Data is not

18. Filling the gap

19. Use your database

20. ROM

23. Ideology
• No global state
• Immutability
• Explicitness
• Leverage your database
• CQRS

24. Overview

25. Overview

26. Gems
• rom – core facili-es
• rom-sql – RDBMS interac-on
• rom-repository – applica-on-level interface

27. Rela%on
• Reads data
• Adapter-specific

28. Rela%ons
class Users < ROM::Relation[:sql]
schema(infer: true)
end

29. Rela%on
class Users < ROM::Relation[:sql]
schema do
attribute :id, Types::Serial
attribute :name, Types::String
end
end

30. Rela%on
class Users < ROM::Relation[:sql]
schema(infer: true)
def by_name(name)
where(name: name)
end
end

31. Command
• Writes data
• Bound to rela0on

32. Command
class CreateUser < ROM::Commands::Create[:sql]
relation :users
register_as :create
result :one
end

33. Container !
rela%ons + commands + DB connec%ons

34. Container !
rom = ROM.container(:sql, 'sqlite::memory')

35. Repository
• Accesses applica+on data
• Receives a container

36. Repository
(ROM::Repository::Root.instance_methods - Object.new.methods).size # => 12
(ROM::Repository::Root.methods - Class.new.methods).size # => 9

37. Repository
(ROM::Repository::Root.instance_methods - Object.new.methods).size # => 12
(ROM::Repository::Root.methods - Class.new.methods).size # => 9
(ApplicationRecord.instance_methods - Object.new.methods).size
(ApplicationRecord.methods - Class.new.methods).size

38. Repository
(ROM::Repository::Root.instance_methods - Object.new.methods).size # => 12
(ROM::Repository::Root.methods - Class.new.methods).size # => 9
(ApplicationRecord.instance_methods - Object.new.methods).size # => 244
(ApplicationRecord.methods - Class.new.methods).size # => 467
TOTAL # => 711

39. Repository
class UserRepo < ROM::Repository[:users]
end

40. Repository
class UserRepo < ROM::Repository[:users]
def list
users.order(:name).to_a
end
end

41. Repository
repo = UserRepo.new(rom)
repo.list # => [#<ROM::Struct[User] id=1 name="Jane">]

42. Changeset
changeset = repo.changeset(name: 'John')
changeset.map(:add_timestamps).to_h
# => {:name=>"John",
# :created_at=>2017-03-18 16:33:19 +0300,
# :updated_at=>2017-03-18 16:33:19 +0300}
repo.create(changeset.map(:add_timestamps))

43. Changeset
changeset = repo.changeset(user.id, name: 'John Doe')
changeset.diff? # => true
changeset.diff # => {:name=>'John Doe'}

44. To the cool stuff! !

45. No global state

46. Many containers

47. Many containers
rom = ROM.container(:sql, DB_URL)
rom_replica = ROM.container(:sql, DB_REPLICA_URL)
repo = UserRepo.new(rom)
read_only_repo = UserRepo.new(rom_replica)

48. Many adapters
rom = ROM.container(
default: [:sql, 'postgres://localhost/rom_repository'],
cache: [:sql, 'sqlite::memory']
) do |c|
c.gateways[:cache].create_table :counters do
primary_key :id
column :user_id, Integer
column :type, String
column :value, Integer
end
end

49. Many adapters
rom = ROM.container(
default: [:sql, 'postgres://localhost/rom_repository'],
cache: [:sql, 'sqlite::memory']
) do |c|
c.gateways[:cache].create_table :counters do
primary_key :id
column :user_id, Integer
column :type, String
column :value, Integer
end
c.relation(:users) do
schema(infer: true)
end
end

50. Many adapters
rom = ROM.container(
default: [:sql, 'postgres://localhost/rom_repository'],
cache: [:sql, 'sqlite::memory']
) do |c|
c.gateways[:cache].create_table :counters do
primary_key :id
column :user_id, Integer
column :type, String
column :value, Integer
end
c.relation(:users) do
schema(infer: true)
end
c.relation(:counters) do
gateway :cache
schema(infer: true)
def for_users(users)
where(user_id: users.pluck(:id))
end
end
end

51. Many adapters
class UserRepo < ROM::Repository[:users]
relations :counters
end
repo = UserRepo.new(rom)

52. Many adapters
repo.
users.
combine(many: {
counters: [repo.counters.for_users, id: :user_id]
}).
where { id <= 2 }.
to_a
=begin
[
#<ROM::Struct[User] id=1 name="Jane" counters=[
#<ROM::Struct[Counter] id=1 user_id=1 type="posts" value=3>,
#<ROM::Struct[Counter] id=2 user_id=1 type="comments" value=512>
]>,
#<ROM::Struct[User] id=2 name="Joe" counters=[]>
]
=end

53. No N+1!
I, [2017-03-18T19:37:14 #6638] (0.000765s) SELECT "id", "name" FROM "users" WHERE ("id" <= 2) ORDER BY "users"."id"
I, [2017-03-18T19:37:14 #6638] (0.000172s) SELECT `id`, `user_id`, `type`, `value` FROM `counters` WHERE (`user_id` IN (1, 2)) ORDER BY `counters`.`id`

54. One database
repo.aggregate(:counters).where { id <= 2 }.to_a

55. Moar backends
• rom-yaml
• rom-h*p
• rom-mongo
• rom-couchdb
• rom-ka3a
• rom-redis
• rom-git

56. Plans
ROM 4.0 roadmap
• More powerful mapping facili2es
• Cross-adapter associa2ons
• Move a bunch of features from rom-repository to rom-core
• Auto-migra2ons in rom-sql ✨

57. Help
• Give it a try
• Use different adapters
• Docs
• Contribute

58. Spasibo