associations: mechanics (esaas §5.3) © 2013 armando fox & david patterson, all rights reserved

Associations: Mechanics(ESaaS §5.3)

© 2013 Armando Fox & David Patterson, all rights reserved

How Does It Work?

• Models must have attribute for foreign key of owning object– e.g., movie_id in reviews table

• ActiveRecord manages this field in both database & in-memory AR object

• Don’t manage it yourself!– Harder to read– May break if database schema doesn’t follow

Rails conventions

Rails Cookery #4

To add a one-to-many association:

1.Add has_many to owning model and belongs_to to owned model

2.Create migration to add foreign key to owned side that references owning side

3.Apply migration

4.rake db:test:prepare to regenerate test database schema

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END

We will get a database error when trying to save a Review

We will have no way of determining which movie a given review is associated withAll of the above

We can say movie.reviews, but review.movie won’t work

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Suppose we have setup the foreign key movie_id in reviews table. If we then add has_many :reviews to Movie, but forget to put belongs_to :movie in Review, what happens?

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END

Through-Associations(ESaaS §5.4)

© 2013 Armando Fox & David Patterson, all rights reserved

Many-to-Many Associations

• Scenario: Moviegoers rate Movies– a moviegoer can have many

reviews– but a movie can also have many reviews

• Why can’t we use has_many & belongs_to?• Solution: create a new AR model to model

the multiple association

Many-to-Many

moviegoer: has_many :reviews movie: has_many :reviews review: belongs_to :moviegoer belongs_to :movie

How to get all movies reviewed by some moviegoer?

reviews

moviegoer_id

movie_id

number

movies

id

...

moviegoer

id

has_many :through

moviegoer: has_many :reviewshas_many :movies, :through => :reviews

movie: has_many :reviews has_many :moviegoers, :through => :reviews

reviews: belongs_to :moviegoer belongs_to :movie

reviews

moviegoer_id

movie_id

...

movies

id

...

moviegoers

id

Through

• Now you can do:@user.movies # movies rated by user@movie.users # users who rated this movie

• My potato scores for R-rated movies@user.reviews.select { |r| r.movie.rating == 'R' }

has_many :through

@user.moviesSELECT * FROM movies JOIN moviegoers ON reviews.moviegoer_id = moviegoers.id JOIN movies ON reviews.movie_id = movies.id

reviews

moviegoer_id

movie_id

...

movies

id

...

moviegoers

id

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END

r = m.reviews.build(:potatoes => 5)r.save!

m.reviews << Review.new(:potatoes=>5)m.save!

All will work

Review.create!(:movie_id=>m.id, :potatoes=>5)☐

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Which of these, if any, is NOT a correct way of saving a new association, given m is an existing movie:

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END

Shortcut: Has and Belongs to Many (habtm)

• join tables express a relationship between existing model tables using FKs

• Join table has no primary key• because there’s no object being represented! movie has_and_belongs_to_many :genres genre has_and_belongs_to_many :movies@movie.genres << Genre.find_by_name('scifi')

genres

id

description

movies

id

name

...etc.

genres_movies

genre_id

movie_id

http://pastebin.com/tTVGtNLx

Rules of Thumb

• If you can conceive of things as different real-world objects, they should probably be distinct models linked through an association

• If you don’t need to represent any other aspect of a M-M relationship, use habtm

• Otherwise, use has_many :through

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HABTM Naming Conventions

M-M relationship naming convention: if a Bar

has_and_belongs_to_many :foos

then a Foohas_and_belongs_to_many :bars

and the database table is the plural AR names in alphabetical order

bars_foos

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END

Faculty HABTM Students,Students HABTM Faculty

Faculty belongs-to appointment,Student belongs-to appointmentFaculty has-many appointments, through Students

Faculty has-many appointments, Student has-many appointments

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We want to model students having appointments with faculty members. Our model would include which relationships:

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END

RESTful Routes for Associations(ESaaS §5.5)

© 2013 Armando Fox & David Patterson, all rights reserved

Creating/Updating Through-Associations

• When creating a new review, how to keep track of the movie and moviegoer with whom it will be associated?– Need this info at creation time– But route helpers like new_movie_path (provided

by resources :movies in routes file) only “carry around” the ID of the model itself

Nested RESTful Routes

in config/routes.rb:resources :moviesbecomesresources :movies do resources :reviewsend

Nested Route: access reviews by going ”through” a movie

Nested RESTful Routes

available as params[:movie_id]available as params[:id]

ReviewsController#create

# POST /movies/1/reviews# POST /movies/1/reviews.xmldef create # movie_id because of nested route @movie = Movie.find(params[:movie_id]) # build sets the movie_id foreign key automatically @review = @movie.reviews.build(params[:review])

if @review.save flash[:notice] = 'Review successfully created.' redirect_to(movie_reviews_path(@movie)) else render :action => 'new' endend

ReviewsController#new

# GET /movies/1/reviews/newdef new # movie_id because of nested route @movie = Movie.find(params[:movie_id]) # new sets movie_id foreign key automatically @review ||= @movie.reviews.new @review = @review || @movie.reviews.newend

• Another possibility: do it in a before-filter before_filter :lookup_moviedef lookup_movie @movie = Movie.find_by_id(params[:movie_id]) || redirect_to movies_path, :flash => {:alert => "movie_id not in params"}end

Views

%h1 Edit

= form_tag movie_review_path(@movie,@review), :method => :put do |f|

...Will f create form fields for a Movie or a Review?

= f.submit "Update Info"

= link_to 'All reviews for this movie', movie_reviews_path(@movie)

• Remember, these are for convenience. Invariant is: review when created or edited must be associated with a movie.

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END

Yes, but we must declare reviews as a nested resource of moviegoers in routes.rb

No, because there can be only one RESTful route to any particular resourceNo, because having more than one through-association involving Reviews would lead to ambiguity

Yes, it should work as-is because of convention over configuration

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If we also have moviegoer has_many reviews,can we use moviegoer_review_path() as a helper?

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END

DRYing Out Queries with Reusable Scopes

(ESaaS §5.6)

© 2013 Armando Fox & David Patterson, all rights reserved

“Customizing” Associations with Declarative Scopes

• Movies appropriate for kids?• Movies with at least N reviews?• Movies with at least average review of N?• Movies recently reviewed?• Combinations of these?

Scopes Can Be “Stacked”

Movie.for_kids.with_good_reviews(3)

Movie.with_many_fans.recently_reviewed

• Scopes are evaluated lazily!

http://pastebin.com/BW40LAHX

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END

Lines 6-7 only

Line 3 AND lines 6-7

Depends on return value of for_kids

Line 3 only☐

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1 # in controller:2 def good_movies_for_kids3 @m = Movie.for_kids.with_good_reviews(3)4 end5 # in view:6 - @m.each do |movie|7 %p= pretty_print(movie)

Where do database queries happen?

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END

Associations Wrap-Up(ESaaS §5.7-5.9)

© 2013 Armando Fox & David Patterson, all rights reserved

Associations Wrap-Up

• Associations are part of application architecture– provides high-level, reusable association constructs

that manipulate RDBMS foreign keys– Mix-ins allow Associations mechanisms to work

with any ActiveRecord subclass

• Proxy methods provide Enumerable-like behaviors– A many-fold association quacks like an Enumerable– Proxy methods are an example of a design pattern

• Nested routes help you maintain associations RESTfully - but they’re optional, and not magic

Elaboration: DataMapper

• Data Mapper associates separate mapper with each model– Idea: keep mapping independent of particular data store

used => works with more types of databases– Used by Google AppEngine– Con: can’t exploit

RDBMS features tosimplify complexqueries & relationships

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Referential Integrity

• What if we delete a movie with reviews?– movie_id field of those reviews then refers to

nonexistent primary key– another reason primary keys are never recycled

• Various possibilities depending on app...– delete those reviews?has_many :reviews, :dependent => :destroy

– make reviews “orphaned”? (no owner) has_many :reviews, :dependent => :nullify

• Can also use lifecycle callbacks to do other things (e.g., merging)

Testing Referential Integrity

it "should nuke reviews when movie deleted" do @movie = @movie.create!(...)

@review = @movie.reviews.create!(...) review_id = @review.id @movie.destroy

end

lambda { Review.find(review_id) }.should raise_error(ActiveRecord::RecordNotFound)

Advanced Topics

• Single-Table Inheritance (STI) & Polymorphic Associations

• Self-referential has_many :through• Many declarative options on manipulating

associations (like validations) • To learn (much) more:

– http://guides.rubyonrails.org/association_basics.html

– The Rails Way, Chapter 9

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END

Better scalability

Worse scalability

All of the above are possible

to have to write the association methods yourself

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If using the DataMapper pattern and you want to do one-to-many associations, you can expect:

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