© 2019 Martin J. Dürst 青山学院大学

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Today's Schedule

• Written test
• Last week's exercises and homework
• Using several models
• Associations between tables
• Migrations
• Today's exercises

Last Lecture's Exercises

• Exercise 9a: Use Scaffold to Create a Todo Application:
  Create application, add data, download data as JSON (with .json extension)
• Exercise 9b: Create an XML API
  Use the builder library to generate XML representations of the data; change the controller to serve XML when requested
• Exercise 9c: Use the JSON API
  Create a program that accesses the JSON API (selected using the Accept: application/json HTTP header) of the application and dowloads data

Using Several Models

• Until now, we have only used a single model per application
• Real applications have more than one model
• Some applications may have more than 100 models
• It is easy to generate separate models with scaffolding
• Models can be associated (connected) to each other

Model Associations Example

• Imagine an application about actors/actresses
• We want to know what movies the actors/actresses played
• We need two models, Actor and Movie
• Each actor/actress can appear in more than one movie
• Each movie has more than one actor appearing
• This association is called many-to-many (many movies to many actors)
• This association is indicated in both models with
  has_and_belongs_to_many :movies (in the Actor model) and
  has_and_belongs_to_many :actors (in the Movie model)
• This association needs an additional database table
  (actors_movies in the case of actors and movies, with columns actor_id and movie_id)

Many-to-One Association

• There are also many-to-one (one-to-many) associations
• Example: Blog posts with comments (each post has many comments, but each comment belongs to exactly one post)
• This association is indicated in the model on the many side (comments) with
  belongs_to :post (singular!)
• This association is indicated in the model on the one side with
  has_many :comments (plural!)
• This association needs an additional database column:
  post_id (integer) on the many side (comments)
• The one side (posts) does not need an additional column:
  The data would not fit into one column
  The data would duplicate data in the post_id column on comments, which is against database rules
• The associated data can be accessed by methods created by ActiveRecord:
  @post.comments (array, plural!)
  @comment.post (singular!)
• Rails translates such methods into database statements

Additional work for Associations

• Associations need more than just an additional table/column and an additional instruction in the models
• Models have to be enhanced to check data (e.g. referential integrity, i.e. the)
• Views have to be enhanced to display additional data or allow the input of additional data
• Controllers have to be enhanced to handle additional data
• Routes and URI may have to be enhanced to express the structure externally
  (e.g. /posts/23/comments/45)
• This lecture will concentrate on views, with some additional work on controllers

Migrations

• Migrations are small Ruby programs that change the database
• Changes to the database can include:
  • Addition or removal of tables
  • Addition or removal of table columns (add_column, remove_column)
  • Change of columns (e.g. add an index, change type, change name,...)
  • Change of data (e.g. change points scale from 1/2/3 to 1/2/3/4/5,...)
• Migrations are created as part of some other generation (e.g. model/scaffold) or with the command
  rails generate migration MigrationName
• Migrations are in the directory db/migrate

Executing Migrations

• Migration names consist of a datetime part and a human-readable part
  (e.g.: 20190617010324_add_points_to_reviews.rb)
• Migrations are executed in datetime order
• rails db:migrate will execute all pending migrations
• Rails will produce an error if there are any pending migrations
• Migrations can be reverted (rolled back) with rails db:rollback parameters
• The status of migrations can be shown with rails db:migrate:status

Migration Details

• Each migration is a small Ruby program defining a migration class
  (e.g. class AddPointsToReviews < ActiveRecord::Migration[5.2])
• The class is a subclass of ActiveRecord::Migration
• The 5.2 indicates the version of the migration library/DSL
• Usually only one method is defined, the change method
• Method calls inside the change method indicate what has to be changed
  (e.g. add_column)
• These methods indicate the change when applying the migration, but can also be run backwards
  (e.g. add_column actually works as remove_column when rolling back a migration)
• Actions for moving forward and backward can also be given separately

View Partials

• Rails prefers programs to be as DRY (don't repeat yourself) as possible
• Sometimes, there may be pars of views that are very similar
• Example: Form parts of new and edit actions
• Rails provides a mechanism called partials for this purpose
• A partial is a file in a view subdirectory starting with an underscore (_name.html.erb)
• Partials are called with render 'name' in the view

DSLs

• DSL stands for Domain Specific Language
• A domain is an application area (e.g. accounting, migrations,...)
• Ruby is very suited for DSLs, mainly because Ruby syntax lets method calls look like simple commands
• Rails uses Ruby for many DSLs (e.g. validations, associations, migrations,...)
• If a DSL uses Ruby, the full power of a programming language is available when needed
• Rails is a very good example for DSL design
• DSLs are an interesting subject for research

Exercise 10a: Two Tables

• Create a new rails application named cinema.
• Create a scaffold for movies. Each movie has a title (string), a description (text).
• Create a scaffold for reviews. Each review has a content (text) and a reference to the movie being reviewed (movie_id, integer).
• Look at the migration for the reviews, and add foreign_key: true to the relevant field.
• Initialize the database with rails db:migrate.
• Start the server.
• Using a browser, add at least 2 movies and at least 3 reviews to your database. Make description and content short but realistic. Make sure you set the movie number for each review correctly.
• Copy the database file to a file named 10aCinema.sqlite3 and submit it to Moodle (deadline 17:00).

Exercise 10b: Showing Movie Titles for Reviews

• After each step, use the browser to check that there are no errors in the application.
• In the models for movies and reviews, add the necessary associations.
• For the reviews, in the views for the actions show and index, display the movie title instead of the movie number. Make sure you also adjust the labels/legends.
• Copy the view files to 10bShow.html.erb and 10bIndex.html.erb, and submit them to Moodle (deadline 17:30).

Exercise 10c: Showing Reviews for Movies

• In the view that shows a movie, add code to display all the reviews.
• Hint: You can reuse the code from the index view of the reviews.
• Keep the links for show and edit, but remove the links for destroy.
• Copy the view file to 10cShow.html.erb and submit it to Moodle (deadline 17:50).

Exercise 10d: Add Points to Reviews

• Create a migration that adds review points to reviews. Use the name AddPointsToReviews.
• Edit the migration file to actually add a points column to the reviews table, with type integer.
• Update the database with rails db:migrate.
• Update the reviews controller to permit a points parameter.
• Add a field for entering review points to the new and edit action views by editing the file _form.html.erb.
• Add points to all your reviews. Use points between 5 (very good) and 1 (very bad).
• Add a field or column for points to all the views where reviews are shown.
• Copy the following files to submit them to Moodle: Copy _form.html.erb to 10d_form.html.erb. Copy the reviews controller to 10dReviewsController.rb. Copy movies/show.html.erb to 10dMoviesShow.html.erb. Copy reviews/index.html.erb to 10dReviewsIndex.html.erb (deadline 18:30).

Homework

(no need to submit)

• Write up ideas about how to make the cinema application more convenient, and how to make it safer (avoid wrong data).
• Check out the associations between tables that Rails provides. Write up examples of where to use them.
• Write up ideas about an application you would want to create yourself.