MV and rails

1. MVC Model Associations
https://flic.kr/p/dxGXD

2. Tests
Migrations
Ye Olde
Internet
Model
DB
Server
Router
View
Browser
Controller

3. Ye Olde
Internet
Server
Router Controller
View
Browser
Model
DB
Tests
Migrations

4. Ye Olde
Internet
Server
Router Controller
View
Browser
Model
DB
MVC Model Review
What key capabilities do Rails
model classes provide?

5. Ye Olde
Internet
Server
Router Controller
View
Browser
Model
DB
MVC Model Review
What key capability do Rails
model classes provide?
CRUD persistent data

6. Ye Olde
Internet
Server
Router Controller
View
Browser
Model
DB
MVC Model Review
How do you create Rails
model classes?

7. Ye Olde
Internet
Server
Router Controller
View
Browser
Model
DB
MVC Model Review
How do you create Rails
model classes?
Generate DB migrations and
model classes with
“rails g model …”
then customize

8. Ye Olde
Internet
Server
Router Controller
View
Browser
Model
DB
MVC Model Review
What purpose do migrations
serve?

9. Ye Olde
Internet
Server
Router Controller
View
Browser
Model
DB
MVC Model Review
What purpose do migrations
serve?
Setup tables in DB via, e.g.,
“rails db:migrate” command

10. MVC Model Review
What DB table would go with this model class?
first_name : string
last_name : string
year : integer
Author
Model Class: Database Table:
???

11. MVC Model Review
What DB table would go with this model class?
id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
first_name : string
last_name : string
year : integer
Author
Model Class: Database Table:

12. MVC Model Review
What DB table would go with this model class?
id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
first_name : string
last_name : string
year : integer
Author
Model Class: Database Table:
Recall Rails ORM
(Object-Relational Mapping)

13. MVC Model Review
What Author methods does Rails
provide “for free”?
first_name : string
last_name : string
year : integer
Author
Model Class:

14. MVC Model Review
What Author methods does Rails
provide “for free”?
first_name : string
last_name : string
year : integer
Author
Model Class: Many methods:
• Author.create / Author.create!
• Author.find
• Author.find_by
• Author.all
• Getters/Setters for attributes:
⎼ Author#first_name
⎼ Author#first_name=
• Author#save / Author#save!
• Author#destroy
• Author#valid?

15. id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
first_name : string
last_name : string
year : integer
Author
Model Class:
Limitation so far:
Insular model classes/tables
Database Table:

16. id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
first_name : string
last_name : string
year : integer
Author
Model Class:
Imagine an author-profile class
Database Table:
birthplace : string
bio : text
awards : text
AuthorProfile
id birthplace bio awards
23 Saint Petersburg Rand was born… Prometheus Award, …
42 New York City He was the son of… Shark Conservation, …
author_profiles

17. What if you want inter-class relationships?
first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
???
For example, you might want to say:

18. Rails Model Associations
Topics:
• OO Class Associations
• Rails ORM
• How to code in Rails

19. Rails Model Supports Three Associations
A B
1
has ▶︎ one to one
(has one / belongs to one)
A B
*
has ▶︎ one to many
(has many / belongs to one)
A B
* many to many
(join table)
1
1
*
AB

20. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Class Association Notation

21. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Class Association Notation
Class Association

22. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Class Association Notation
Association Name
Triangle indicates reading direction
(i.e., Author has AuthorProfile)

23. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Class Association Notation
Role Names
Like attribute/method name
Enables this method call:
@author.profile.birthplace

24. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Class Association Notation
Multiplicities

25. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Class Association Notation
Multiplicities
Each Author has 1 AuthorProfile

26. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Class Association Notation
Multiplicities
Each Author has 1 AuthorProfile
Each AuthorProfile belongs to 1 Author

27. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Class Association Notation
Multiplicities
Each Author has 1 AuthorProfile
Each AuthorProfile belongs to 1 Author
Zero-or-More Multiplicities:
A B
*
has
Each A has 0 or more Bs
A B
0..*
has
Each A has 0 or more Bs

28. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Class Association Notation
first_name = "Ayn"
last_name = "Rand"
year_born = 1905
: Author
first_name = "Peter"
last_name = "Benchley"
year_born = 1940
: Author
birthplace = "Saint Petersburg"
bio = "Rand was born..."
awards = "Prometheus Award, …"
: AuthorProfile
birthplace = "New York City"
bio = "He was the son of..."
awards = " Shark Conservation, … "
: AuthorProfile
Associations manifest as links in object diagrams
author profile
author profile

29. Class Association Notation
name : string
Physician
name : string
Patient
*
physician
*
patient
appointment_date : datetime
Appointment
Association Class
Each instance is an association link
Can add additional data to each link (e.g., date)

30. name : string
Physician
name : string
Patient
*
physician
*
patient
appointment_date : datetime
Appointment
Enables code like this:
Class Association Notation

31. name : string
Physician
name : string
Patient
*
physician
*
patient
appointment_date : datetime
Appointment
Class Association Notation
name = "Dr. Hibbert"
: Physician
name = "Homer Simpson"
: Patient
name = "Marge Simpson"
: Patient
appointment_date = ...
: Appointment
appointment_date = ...
: Appointment
physician
patient
physician
patient
How many-to-many links look in object diagrams:

32. Rails Model Associations
Topics:
• OO Class Associations
• Rails ORM
• How to code in Rails

33. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
To map this association to relational DB...
first_name = "Ayn"
last_name = "Rand"
year_born = 1905
: Author
first_name = "Peter"
last_name = "Benchley"
year_born = 1940
: Author
birthplace = "Saint Petersburg"
bio = "Rand was born..."
awards = "Prometheus Award, …"
: AuthorProfile
birthplace = "New York City"
bio = "He was the son of..."
awards = " Shark Conservation, … "
: AuthorProfile
author profile
author profile

34. id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
id birthplace bio awards
23 Saint Petersburg Rand was born… Prometheus Award, …
42 New York City He was the son of… Shark Conservation, …
author_profiles
Requires inter-table references

35. Rails ORM Varies by Type of Association
A B
1
has ▶︎ one to one
(has one / belongs to one)
A B
*
has ▶︎ one to many
(has many / belongs to one)
A B
* many to many
(join table)
1
1
*
AB

36. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Has One / Belongs To One Example
id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
id author _id birthplace bio awards
23 1 Saint Petersburg Rand was born… Prometheus Award, …
42 2 New York City He was the son of… Shark Conservation, …
author_profiles
Rails ORM DB Tables

37. first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
Has One / Belongs To One Example
id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
id author _id birthplace bio awards
23 1 Saint Petersburg Rand was born… Prometheus Award, …
42 2 New York City He was the son of… Shark Conservation, …
author_profiles
Rails ORM DB Tables

38. id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
id author _id birthplace bio awards
23 1 Saint Petersburg Rand was born… Prometheus Award, …
42 2 New York City He was the son of… Shark Conservation, …
author_profiles
Primary versus Foreign Keys
Primary Key: Uniquely identifies each record in table

39. id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
id author _id birthplace bio awards
23 1 Saint Petersburg Rand was born… Prometheus Award, …
42 2 New York City He was the son of… Shark Conservation, …
author_profiles
Primary versus Foreign Keys
Primary Key: Uniquely identifies each record in table
Foreign Key: Reference to primary key in another table

40. id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
id author _id birthplace bio awards
23 1 Saint Petersburg Rand was born… Prometheus Award, …
42 2 New York City He was the son of… Shark Conservation, …
author_profiles
Primary versus Foreign Keys
Primary Key: Uniquely identifies each record in table
Foreign Key: Reference to primary key in another table

41. How to Set Up Association in Rails
1. Generate migration that adds reference (foreign
key) column to appropriate table
2. Add has_one/belongs_to declarations to
model class
See forthcoming demo for details

42. Rails Relationship Support
• has one / belongs to one
• has many / belongs to one
• join table

43. Has Many / Belongs to One Example
first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
title : string
year : integer
summary : text
Book
How to model authorship?
(assume 1 author per book)

44. Has Many / Belongs to One Example
first_name : string
last_name : string
year : integer
Author
birthplace : string
bio : text
awards : text
AuthorProfile
has
1
author
1
profile
title : string
year : integer
summary : text
Book
How to model authorship?
(assume 1 author per book)
has
1 author
* book

45. Has Many / Belongs to One Example
first_name : string
last_name : string
year : integer
Author
title : string
year : integer
summary : text
Book
What should the ORM be?
(hint: authors table unchanged)
has
1 author
* book
id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors

46. Has Many / Belongs to One Example
first_name : string
last_name : string
year : integer
Author
title : string
year : integer
summary : text
Book
What should the ORM be?
(hint: authors table unchanged)
has
1 author
* book
id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
id title year summary author_id
72 The Fountainhead 1943 Individualistic architect… 1
98 Atlas Shrugged 1957 Dystopian USA… 1
99 Jaws 1974 Shark!... 2
books

47. Has Many / Belongs to One Example
first_name : string
last_name : string
year : integer
Author
title : string
year : integer
summary : text
Book
What should the ORM be?
(hint: authors table unchanged)
has
1 author
* book
id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
id title year summary author_id
72 The Fountainhead 1943 Individualistic architect… 1
98 Atlas Shrugged 1957 Dystopian USA… 1
99 Jaws 1974 Shark!... 2
books

48. Has Many / Belongs to One Example
first_name : string
last_name : string
year : integer
Author
title : string
year : integer
summary : text
Book
has
1 author
* book
id first_name last_name year
1 Ayn Rand 1905
2 Peter Benchley 1940
authors
id title year summary author_id
72 The Fountainhead 1943 Individualistic architect… 1
98 Atlas Shrugged 1957 Dystopian USA… 1
99 Jaws 1974 Shark!... 2
books
Rails generates books method
that returns list of books

49. How to Set Up in Rails
• Essentially the same as has_one
• Again, see the demo

50. Rails Relationship Support
• has one / belongs to one
• has many / belongs to one
• join table

51. Example Many-to-Many Association
name : string
Physician
name : string
Patient
*
physician
*
patient
appointment_date : datetime
Appointment

52. Join Table Example
From http://guides.rubyonrails.org/v4.2.0/association_basics.html

53. Join Table Example
Join table
From http://guides.rubyonrails.org/v4.2.0/association_basics.html

54. Join Table Example
References
(aka foreign keys)
From http://guides.rubyonrails.org/v4.2.0/association_basics.html

55. Join Table Example: Model Class
through enables this sort of thing:
From http://guides.rubyonrails.org/v4.2.0/association_basics.html

56. How to Set Up in Rails
• See the Model Association demos
• For more info:
– http://guides.rubyonrails.org/v6.1.0/association_basics.html
– http://api.rubyonrails.org/v6.1.0/classes/ActiveRecord/Asso
ciations/ClassMethods.html

57. Summary
• Model associations
• Foreign keys
• Rails ORM for assoc.
• Has-one/belong-to-one
• Has-many/belongs-to-one
• Many-to-many join table
http://flic.kr/p/aCLor3