The document provides an overview of effective ActiveRecord usage in Ruby on Rails. It reviews models, associations like has_many and belongs_to, autosave functionality, and authorization patterns when creating records. Some key points covered include using autosave to automatically save associated records, inspecting SQL to ensure it looks as expected, and separating build from create when authorizing user actions.

1. Effective ActiveRecord
Sam Goldman
@nontrivialzeros
http://github.com/samwgoldman
sam@smartlogic.io
Wednesday, December 18, 13

2. Review: Models
class User < ActiveRecord::Base
end
foo = User.find(1)
foo.name # "Foo"
foo.email # "foo@example.com"
bar = User.find(2)
bar.name # "Bar
bar.email # "bar@example.com"
Wednesday, December 18, 13
users
id
email
name
1 foo@example.com Foo
2 bar@example.com Bar

3. Review: Has Many
class Project < AR::Base
has_many :members
end
class Member < AR::Base
belongs_to :project
end
foo_project = Project.find(1)
foo_project.name
# "Foo project"
foo_project.members.
map(&:email)
# ["foo@example.com",
# "bar@example.com"]
Wednesday, December 18, 13
projects
id
name
1 Foo project
2 Bar project
members
id project_id
email
1
1
foo@example.com
2
1
bar@example.com
3
2
baz@example.com
4
2
quux@example.com

4. Review: Belongs To
class Project < AR::Base
has_many :members
end
class Member < AR::Base
belongs_to :project
end
foo = Member.find(1)
foo.email
# "foo@example.com"
foo.project.name
# "Foo project"
Wednesday, December 18, 13
projects
id
name
1 Foo project
2 Bar project
members
id project_id
email
1
1
foo@example.com
2
1
bar@example.com
3
2
baz@example.com
4
2
quux@example.com

5. Review: Has Many Through
class User < AR::Base
has_many :members
has_many :projects,
through: :members
end
class Project < AR::Base
has_many :members
end
class Member < AR::Base
belongs_to :user
belongs_to :project
end
foo = User.find(1)
foo.projects.map(&:name)
# ["Foo project",
# "Bar project"]
Wednesday, December 18, 13
users
id
email
name
1 foo@example.com Foo
projects
id
name
1 Foo project
2 Bar project
members
id project_id
user_id
1
1
1
3
2
1

6. Creating Records
project = Project.create(name: "Project")
(0.3ms) BEGIN
SQL (1.5ms) INSERT INTO "projects" ("name") VALUES ($1)
RETURNING "id"
[["name", "Project"]]
(0.4ms) COMMIT
user = User.create(name: "User", email: "user@example.com")
(0.3ms) BEGIN
SQL (1.3ms) INSERT INTO "users" ("email", "name") VALUES ($1, $2)
RETURNING "id"
[["email", "user@example.com"], ["name", "User"]]
(0.4ms) COMMIT
member = Member.create(user: user, project: project)
(0.5ms) BEGIN
SQL (3.7ms) INSERT INTO "members" ("project_id", "user_id") VALUES ($1, $2)
RETURNING "id"
[["project_id", 1], ["user_id", 1]]
(0.3ms) COMMIT
Wednesday, December 18, 13

7. Updating Records
project.update_attributes(name: "Updated Project")
(0.2ms) BEGIN
SQL (0.9ms) UPDATE "projects" SET "name" = $1
WHERE "projects"."id" = 1
[["name", "Updated Project"]]
(0.4ms) COMMIT
user.update_attributes(name: "Updated User")
(0.1ms) BEGIN
SQL (0.9ms) UPDATE "users" SET "name" = $1
WHERE "users"."id" = 1
[["name", "Updated User"]]
(0.4ms) COMMIT
Wednesday, December 18, 13

8. Autosave
class Member < ActiveRecord::Base
belongs_to :user
belongs_to :project
end
project = Project.new(name: "Project")
user = User.new(name: "User", email: "user@example.com")
member = Member.create(user: user, project: project)
Guess the result.
Wednesday, December 18, 13

9. Autosave
class Member < ActiveRecord::Base
belongs_to :user
belongs_to :project
end
project = Project.new(name: "Project")
user = User.new(name: "User", email: "user@example.com")
member = Member.create(user: user, project: project)
(0.4ms) BEGIN
SQL (2.7ms) INSERT INTO "users" ("email", "name") VALUES ($1, $2)
RETURNING "id"
[["email", "user@example.com"], ["name", "User"]]
SQL (1.2ms) INSERT INTO "projects" ("name") VALUES ($1)
RETURNING "id"
[["name", "Project"]]
SQL (3.5ms) INSERT INTO "members" ("project_id", "user_id") VALUES ($1, $2)
RETURNING "id"
[["project_id", 1], ["user_id", 1]]
(0.5ms) COMMIT
Wednesday, December 18, 13

10. Autosave
class Member < ActiveRecord::Base
belongs_to :user
belongs_to :project
end
member = Member.new
member.build_user(name: "User", email: "user@example.com")
member.build_project(name: "Project")
member.save
(0.4ms) BEGIN
SQL (2.7ms) INSERT INTO "users" ("email", "name") VALUES ($1, $2)
RETURNING "id"
[["email", "user@example.com"], ["name", "User"]]
SQL (1.2ms) INSERT INTO "projects" ("name") VALUES ($1)
RETURNING "id"
[["name", "Project"]]
SQL (3.5ms) INSERT INTO "members" ("project_id", "user_id") VALUES ($1, $2)
RETURNING "id"
[["project_id", 1], ["user_id", 1]]
(0.5ms) COMMIT
Wednesday, December 18, 13

11. Autosave
class Member < ActiveRecord::Base
belongs_to :user, autosave: false
belongs_to :project, autosave: false
end
member = Member.new
member.build_user(name: "User", email: "user@example.com")
member.build_project(name: "Project")
member.save
Guess the result.
Wednesday, December 18, 13

12. Autosave
class Member < ActiveRecord::Base
belongs_to :user, autosave: false
belongs_to :project, autosave: false
end
member = Member.new
member.build_user(name: "User", email: "user@example.com")
member.build_project(name: "Project")
member.save
PG::NotNullViolation:
ERROR: null value in column "user_id" violates not-null constraint
(ActiveRecord::StatementInvalid)
DETAIL: Failing row contains (1, null, null).
: INSERT INTO "members" DEFAULT VALUES RETURNING "id"
Wednesday, December 18, 13

13. Autosave
class Member < ActiveRecord::Base
belongs_to :user
belongs_to :project
end
member.user.name = "Updated User"
member.project.name = "Updated Project"
member.save
Guess the result.
Wednesday, December 18, 13

14. Autosave
class Member < ActiveRecord::Base
belongs_to :user
belongs_to :project
end
member.user.name = "Updated User"
member.project.name = "Updated Project"
member.save
(0.2ms) BEGIN
(0.2ms) COMMIT
Wednesday, December 18, 13

15. Autosave
class Member < ActiveRecord::Base
belongs_to :user, autosave: true
belongs_to :project, autosave: true
end
member.user.name = "Updated User"
member.project.name = "Updated Project"
member.save
Guess the result.
Wednesday, December 18, 13

16. Autosave
class Member < ActiveRecord::Base
belongs_to :user, autosave: true
belongs_to :project, autosave: true
end
member.user.name = "Updated User"
member.project.name = "Updated Project"
member.save
(0.2ms) BEGIN
SQL (1.1ms) UPDATE "users" SET "name" = $1
WHERE "users"."id" = 1
[["name", "Updated User"]]
SQL (1.2ms) UPDATE "projects" SET "name" = $1
WHERE "projects"."id" = 1
[["name", "Updated Project"]]
(0.4ms) COMMIT
Wednesday, December 18, 13

17. Autosave
class Project < ActiveRecord::Base
has_many :members
end
user = User.new(name: "User", email: "user@example.com")
project = Project.new(name: "Project")
project.members << Member.new(user: user)
project.save
Guess the result.
Wednesday, December 18, 13

18. Autosave
class Project < ActiveRecord::Base
has_many :members
end
user = User.new(name: "User", email: "user@example.com")
project = Project.new(name: "Project")
project.members << Member.new(user: user)
project.save
(0.7ms) BEGIN
SQL (1.6ms) INSERT INTO "projects" ("name") VALUES ($1)
RETURNING "id"
[["name", "Project"]]
SQL (1.2ms) INSERT INTO "users" ("email", "name") VALUES ($1, $2)
RETURNING "id"
[["email", "user@example.com"], ["name", "User"]]
SQL (3.4ms) INSERT INTO "members" ("project_id", "user_id") VALUES ($1, $2)
RETURNING "id"
[["project_id", 1], ["user_id", 1]]
(0.5ms) COMMIT
Wednesday, December 18, 13

19. Autosave
class Project < ActiveRecord::Base
has_many :members
end
user = User.new(name: "User", email: "user@example.com")
project = Project.new(name: "Project")
project.members.build(user: user)
project.save
(0.7ms) BEGIN
SQL (1.6ms) INSERT INTO "projects" ("name") VALUES ($1)
RETURNING "id"
[["name", "Project"]]
SQL (1.2ms) INSERT INTO "users" ("email", "name") VALUES ($1, $2)
RETURNING "id"
[["email", "user@example.com"], ["name", "User"]]
SQL (3.4ms) INSERT INTO "members" ("project_id", "user_id") VALUES ($1, $2)
RETURNING "id"
[["project_id", 1], ["user_id", 1]]
(0.5ms) COMMIT
Wednesday, December 18, 13

20. Autosave
class Project < ActiveRecord::Base
has_many :members, autosave: false
end
user = User.new(name: "User", email: "user@example.com")
project = Project.new(name: "Project")
project.members.build(user: user)
project.save
Guess the result.
Wednesday, December 18, 13

21. Autosave
class Project < ActiveRecord::Base
has_many :members, autosave: false
end
user = User.new(name: "User", email: "user@example.com")
project = Project.new(name: "Project")
project.members.build(user: user)
project.save
(0.4ms) BEGIN
SQL (1.6ms) INSERT INTO "projects" ("name") VALUES ($1)
RETURNING "id"
[["name", "Project"]]
(0.4ms) COMMIT
Wednesday, December 18, 13

22. Inverses
class Project < ActiveRecord::Base
has_many :tasks
end
class Task < ActiveRecord::Base
belongs_to :project
end
project = Project.new(name: "Project")
task = project.tasks.build
project.save
p project.object_id
p task.project.object_id
Guess the result.
Wednesday, December 18, 13

23. Inverses
class Project < ActiveRecord::Base
has_many :tasks
end
class Task < ActiveRecord::Base
belongs_to :project
end
project = Project.new(name: "Project")
task = project.tasks.build
project.save
p project.object_id
p task.project.object_id
Not just an
extra query. Split
brain!
70236648295560
Project Load (1.4ms) SELECT "projects".* FROM "projects"
WHERE "projects"."id" = $1
ORDER BY "projects"."id" ASC
LIMIT 1
[["id", 1]]
70236645304160
Wednesday, December 18, 13

24. Inverses
class Project < ActiveRecord::Base
has_many :tasks, inverse_of: :project
end
class Task < ActiveRecord::Base
belongs_to :project
end
project = Project.new(name: "Project")
task = project.tasks.build
project.save
p project.object_id
p task.project.object_id
Guess the result.
Wednesday, December 18, 13

25. Inverses
class Project < ActiveRecord::Base
has_many :tasks, inverse_of: :project
end
class Task < ActiveRecord::Base
belongs_to :project
end
project = Project.new(name: "Project")
task = project.tasks.build
project.save
p project.object_id
p task.project.object_id
70259515608140
70259515608140
Wednesday, December 18, 13

26. Summary
• Use autosave and inverse associations
• Inspect the generated SQL for sanity
• Avoid explicit transactions
Wednesday, December 18, 13

27. Authorization
class ProjectMembersController < ApplicationController
# POST /project/:project_id/members
def create
member = Member.create(member_params.merge(project_id: project_id))
respond_with member
end
private
def project_id
params.require(:project_id)
end
def member_params
params.require(:member).permit(:user_id)
end
end
Wednesday, December 18, 13

28. Authorization
class ProjectMembersController < ApplicationController
# POST /project/:project_id/members
def create
member = Member.create(member_params.merge(project_id: project_id))
respond_with member
end
Anyone can
def project_id
add any user to any
params.require(:project_id)
end
project!
private
def member_params
params.require(:member).permit(:user_id)
end
end
Wednesday, December 18, 13

29. Authorization
class ProjectMembersController < ApplicationController
# POST /project/:project_id/members
def create
member = current_user.create_project_member(project_id, member_params)
respond_with member
end
private
def project_id
params.require(:project_id)
end
def member_params
params.require(:member).permit(:user_id)
end
end
class User < ActiveRecord::Base
has_many :members
has_many :projects, through: :members
May only add
members to my own
projects.
def create_project_member(project_id, member_params)
project = projects.find(project_id)
project.members.create(member_params)
end
end
Wednesday, December 18, 13

30. Authorization
class ProjectMembersController < ApplicationController
# POST /project/:project_id/members
def create
member = current_user.build_project_member(project_id, member_params)
member.save
respond_with member
end
private
def project_id
params.require(:project_id)
end
Separate build vs.
create
def member_params
params.require(:member).permit(:user_id)
end
end
class User < ActiveRecord::Base
has_many :members
has_many :projects, through: :members
def build_project_member(project_id, member_params)
project = projects.find(project_id)
project.members.build(member_params)
end
end
Wednesday, December 18, 13

31. Authorization
class ProjectMembersController < ApplicationController
# POST /project/:project_id/members
def create
member = current_user.build_project_member(project_id, member_params)
member.save
respond_with member
end
private
def project_id
params.require(:project_id)
end
def member_params
params.require(:member).permit(:user_id)
end
end
What if I am
not a member of this
class User < ActiveRecord::Base
has_many :members
has_many :projects, through: :members
project?
def build_project_member(project_id, member_params)
project = projects.find(project_id)
project.members.build(member_params)
end
end
Wednesday, December 18, 13

32. Authorization
Couldn't find Project with id=1
(ActiveRecord::RecordNotFound)
Wednesday, December 18, 13

33. Authorization
class User < ActiveRecord::Base
has_many :members, inverse_of: :user
has_many :projects, through: :members
def build_project_member(project_id, member_params)
project = projects.find_one(project_id)
if project
project.members.build(member_params)
end
end
end
Wednesday, December 18, 13

34. Authorization
class User < ActiveRecord::Base
has_many :members, inverse_of: :user
has_many :projects, through: :members
def member(project_id)
members.find_by(project_id: project_id)
end
def build_project_member(project_id, member_params)
member = member(project_id)
if member
member.build_project_member(member_params)
end
end
end
class Member < ActiveRecord::Base
belongs_to :user, inverse_of: :members
belongs_to :project, inverse_of: :members
def build_project_member(member_params)
project.members.build(member_params)
end
end
Wednesday, December 18, 13

35. Authorization
class User < ActiveRecord::Base
has_many :members, inverse_of: :user
has_many :projects, through: :members
def member(project_id)
members.find_by(project_id: project_id)
end
def build_project_member(project_id, member_params)
member = member(project_id)
if member
member.build_project_member(member_params)
end
end
end
class Member < ActiveRecord::Base
belongs_to :user, inverse_of: :members
belongs_to :project, inverse_of: :members
def build_project_member(member_params)
if role == "admin"
project.members.build(member_params)
end
end
end
Wednesday, December 18, 13

36. Authorization
class ProjectMembersController < ApplicationController
# POST /project/:project_id/members
def create
member = current_user.build_project_member(project_id, member_params)
if member.nil?
# handle error
else
member.save
respond_with member
end
end
private
def project_id
params.require(:project_id)
end
def member_params
params.require(:member).permit(:user_id)
end
end
Wednesday, December 18, 13

37. Authorization
class ProjectMembersController < ApplicationController
# POST /project/:project_id/members
def create
member = current_user.build_project_member(project_id, member_params)
if member.nil?
# handle error
else
member.save
respond_with member
end
end
private
Which error
happened?
def project_id
params.require(:project_id)
end
def member_params
params.require(:member).permit(:user_id)
end
end
Wednesday, December 18, 13

38. Authorization
Failure = Struct.new(:error) do
def success?
false
end
end
Success = Struct.new(:value) do
def success?
true
end
end
class Member < ActiveRecord::Base
belongs_to :user, inverse_of: :members
belongs_to :project, inverse_of: :members
def build_project_member(member_params)
if role == "admin"
Success.new(project.members.build(member_params))
else
Failure.new(:not_authorized)
end
end
end
Wednesday, December 18, 13

39. Authorization
class User < ActiveRecord::Base
has_many :members, inverse_of: :user
has_many :projects, through: :members
def member(project_id)
member = members.find_by(project_id: project_id)
if member
Success.new(member)
else
Failure.new(:member_not_found)
end
end
def build_project_member(project_id, member_params)
result = member(project_id)
if result.success?
result.value.build_project_member(member_params)
else
result
end
end
end
Wednesday, December 18, 13

40. Authorization
class ProjectMembersController < ApplicationController
# POST /project/:project_id/members
def create
result = current_user.build_project_member(project_id, member_params)
if result.success?
member = result.value
member.save
respond_with member
else
result.error # handle error
end
end
private
def project_id
params.require(:project_id)
end
def member_params
params.require(:member).permit(:user_id)
end
end
Wednesday, December 18, 13

41. Authorization
project = Project.create(name: "Project")
alice = User.create(name: "Alice", email: "alice@example.com")
bob = User.create(name: "Bob", email: "bob@example.com")
p alice.build_project_member(project.id, {
user_id: bob.id,
role: "member"
})
#<struct Failure error=:member_not_found>
Wednesday, December 18, 13

42. Authorization
project = Project.create(name: "Project")
alice = User.create(name: "Alice", email: "alice@example.com")
bob = User.create(name: "Bob", email: "bob@example.com")
alice.members.create(project: project, role: "member")
p alice.build_project_member(project.id, {
user_id: bob.id,
role: "member"
})
#<struct Failure error=:not_authorized>
Wednesday, December 18, 13

43. Authorization
project = Project.create(name: "Project")
alice = User.create(name: "Alice", email: "alice@example.com")
bob = User.create(name: "Bob", email: "bob@example.com")
alice.members.create(project: project, role: "admin")
p alice.build_project_member(project.id, {
user_id: bob.id,
role: "member"
})
#<struct Success value=#<Member user_id: 2, project_id: 1, role: "member">>
Wednesday, December 18, 13

44. Summary
• Use the relations
• Move beyond ActiveRecord’s API
• Use result objects to represent possible
failures
• Separate building vs. creating APIs
Wednesday, December 18, 13

45. Refactoring
class User < ActiveRecord::Base
has_many :members
has_many :projects, through: :members
def member(project_id)
member = members.find_by(project_id: project_id)
if member
Success.new(member)
else
Failure.new(:member_not_found)
end
end
def build_project_member(project_id, member_params)
result = member(project_id)
if result.success?
result.value.build_project_member(member_params)
else
result
end
end
end
Wednesday, December 18, 13

46. Refactoring
class User < ActiveRecord::Base
has_many :members
has_many :projects, through: :members
def member(project_id)
member = members.find_by(project_id: project_id)
if member
Success.new(member)
else
Failure.new(:member_not_found)
end
end
def build_project_member(project_id, member_params)
result = member(project_id)
if result.success?
result.value.build_project_member(member_params)
else
result
end
end
end
Wednesday, December 18, 13
We need a way to
combine results.

47. Refactoring
Failure = Struct.new(:error) do
def success?
false
end
def map
self
end
def bind
self
end
end
Wednesday, December 18, 13
Success = Struct.new(:value) do
def success?
true
end
def map
Success.new(yield value)
end
def bind
yield value
end
end

48. Refactoring
class User < ActiveRecord::Base
has_many :members
has_many :projects, through: :members
def member(project_id)
member = members.find_by(project_id: project_id)
if member
Success.new(member)
else
Failure.new(:member_not_found)
end
end
Build compound
results.
def build_project_member(project_id, member_params)
member(project_id).bind do |member|
member.build_project_member(member_params)
end
end
end
Wednesday, December 18, 13

49. Serialize
class Member < ActiveRecord::Base
belongs_to :user, inverse_of: :members
belongs_to :project, inverse_of: :members
def build_project_member(member_params)
if role == "admin"
Success.new(project.members.build(member_params))
else
Failure.new(:not_authorized)
end
end
end
Wednesday, December 18, 13

50. Serialize
class Member < ActiveRecord::Base
belongs_to :user, inverse_of: :members
belongs_to :project, inverse_of: :members
serialize :role, Role
def build_project_member(member_params)
role.build_project_member(project, member_params)
end
end
Wednesday, December 18, 13

51. Serialize
class Role
Unknown = Object.new
def Unknown.name
nil
end
MAP = {}
MAP.default = Unknown
def self.load(name)
MAP[name]
end
def self.dump(role)
role.name
end
attr_reader :name
def initialize(name, &block)
@name = name
instance_eval(&block)
MAP[name] = self
end
end
Wednesday, December 18, 13

52. Serialize
class Role
Admin = Role.new("admin") do
def build_project_member(project, member_params)
Success.new(project.members.build(member_params))
end
end
Member = Role.new("member") do
def build_project_member(project, member_params)
Failure.new(:not_authorized)
end
end
Null = Role.new(nil) do
def build_project_member(project, member_params)
Failure.new(:missing_role)
end
end
def Unknown.build_project_member(project, member_params)
Failure.new(:unknown_role)
end
end
Wednesday, December 18, 13

53. Authorization
class ProjectMembersController < ApplicationController
# POST /project/:project_id/members
def create
result = current_user.build_project_member(project_id, member_params)
if result.success?
member = result.value
member.save
respond_with member
else
result.error # handle error
end
end
private
def project_id
params.require(:project_id)
end
def member_params
member_params = params.require(:member).permit(:user_id, :role)
role = Role.load(member_params[:role].presence)
member_params.merge(:role => role)
end
end
Wednesday, December 18, 13

54. Questions?
http://smartlogic.io
http://twitter.com/smartlogic
http://github.com/smartlogic
http://facebook.com/smartlogic
Wednesday, December 18, 13