Associations: mechanics (ESaaS 5.3)"

Size: px

Start display at page:

Download "Associations: mechanics (ESaaS 5.3)""

Benjamin Cox
5 years ago
Views:

2 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"

3 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"

4 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?" " We can say movie.reviews, but review.movie won't work" " 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 with" " All of the above" 4"

6 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!

7 Many-to-many" moviegoer" id" reviews" moviegoer_id" movie_id" number" movies" id"..." moviegoer: has_many :reviews movie: has_many :reviews review: belongs_to :moviegoer belongs_to :movie How to get all movies reviewed by some moviegoer? "

8 has_many :through moviegoers" id" reviews" moviegoer_id" movie_id"..." movies" id"..." moviegoer: has_many :reviews has_many :movies, :through => :reviews movie: has_many :reviews has_many :moviegoers, :through => :reviews" reviews: belongs_to :moviegoer belongs_to :movie

9 Now you can do:" # movies rated by # users who rated this movie My potato scores for R-rated { r r.movie.rating == 'R' }

10 has_many :through moviegoers" id" reviews" moviegoer_id" movie_id"..." movies" SELECT * FROM movies JOIN moviegoers ON reviews.moviegoer_id = moviegoers.id JOIN movies ON reviews.movie_id = movies.id

11 Which of these, if any, is NOT a correct way of saving a new association, given m is an existing movie:" Review.create!(:movie_id=>m.id, :potatoes=>5) r = m.reviews.build(:potatoes => 5) r.save! m.reviews << Review.new(:potatoes=>5) m.save! All will work" 11"

13 A 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 << Genre.find_by_name('scifi') genres" id" description" genres_movies" genre_id" movie_id" movies" id" name"...etc."

14 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 14 14"

15 HABTM Naming Conventions" " M-M relationship naming convention: " if a Bar " has_and_belongs_to_many :foos" then a Foo" has_and_belongs_to_many :bars" and the database table is the " plural AR names in alphabetical order" bars_foos" 15

16 We want to model students having appointments with faculty members. Our model would include which relationships:" Faculty has-many appointments, Student has-many appointments" Faculty HABTM Students," Students HABTM Faculty" Faculty belongs-to appointment," Student belongs-to appointment" Faculty has-many appointments, " through Students" 16"

18 Creating/updating throughassociations" 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"

19 Nested RESTful Routes" " in config/routes.rb: resources :movies becomes resources :movies do resources :reviews end " Nested Route: access reviews by going through a movie"

20 Nested RESTful Routes" available as params[:movie_id] available as params[:id]

21 ReviewsController#create # POST /movies/1/reviews # POST /movies/1/reviews.xml def create # movie_id because of nested = Movie.find(params[:movie_id]) # build sets the movie_id foreign key flash[:notice] = 'Review successfully created.' redirect_to(movie_reviews_path(@movie)) else render :action => 'new' end end

22 ReviewsController#new" # GET /movies/1/reviews/new def new # movie_id because of nested = Movie.find(params[:movie_id]) # new sets movie_id foreign key = = end Another possibility: do it in a before-filter " before_filter :lookup_movie def = Movie.find_by_id(params[:movie_id]) redirect_to movies_path, :flash => {:alert => "movie_id not in params"} end

23 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."

24 If we also have moviegoer has_many reviews," can we use moviegoer_review_path() as a helper?" " Yes, it should work as-is because of convention over configuration" 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 resource" No, because having more than one through-association involving Reviews would lead to ambiguity" 24"

26 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?!

27 Scopes Can Be Stacked " Movie.for_kids.with_good_reviews(3) Movie.with_many_fans.recently_reviewed Scopes are evaluated lazily!"

28 1 # in controller: 2 def good_movies_for_kids = Movie.for_kids.with_good_reviews(3) 4 end 5 # in view: 6 do movie 7 %p= pretty_print(movie) Where do database queries happen?" "Line 3 only" Lines 6-7 only" Line 3 AND lines 6-7" Depends on return value of for_kids" 28"

30 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 =>

31 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 to simplify complex queries & relationships" 31"

32 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 (eg, merging)"

33 Testing Referential Integrity" it "should nuke reviews when movie deleted" lambda { Review.find(review_id) }.should raise_error(activerecord::recordnotfound)

34 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:" association_basics.html " The Rails Way, Chapter 9"

35 If using the DataMapper pattern and you want to do one-to-many associations, you can expect:" " to have to write the association methods yourself" Better scalability" Worse scalability" All of the above are possible" 35"

A shortcut: has and belongs to many (habtm)

A 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! ttvgtnlx