Lecture 6. Active Record Associations 1 / 34

Transcription

1 Lecture 6 Active Record Associations 1 / 34

2 Midterm Course Evaluations 2 / 34

3 Learn HTML You're going to be writing your own views in the next HW Make sure to familiarise yourself with the syntax and the basic structure of HTML (i.e. it's a hierarchical structure). 3 / 34

4 Homeworks 4 & 5 Will be graded by Monday 4 / 34

5 What are migrations? Vote at PollEv.com/CIS / 34

6 Generators 6 / 34

7 More generators Controller Generator: rails g controller controller_name action1 action2 Scaffold Controller Generator: rails g scaffold_controller model_name col1:type col2:type 7 / 34

8 Controller Generator You pass the generator the name of the controller and the actions that you want inside of it Generates the controller file with the actions that you pass along with it Also generates view files for the actions Should be used to create a controller file with no associated model e.g. the WelcomeController doesn't have an associated model rails g controller welcome index 8 / 34

9 Sca old Controller Generator You pass the generator the name of the controller and the columns in the table Will generate the controller with the 7 RESTful actions Also generates the views for each of these actions Should be used when you want the 7 RESTful routes and a model is associated rails g scaffold_controller note notebook:references title:st 9 / 34

10 before_action You'll notice a before_action in scaffold generated controllers Takes a method as an argument and calls that method before every action in the controller # To call the method before all actions in the controller: before_action :method_name # To only call the method before select actions: before_action :method_name, only: [:action1, :action2,...] # To call the method before all actions except a select few: before_action :method_name, except: [:action1, :action2,...] # Example: before_action :set_note, only: [:show, :edit, :update, :destroy] 10 / 34

11 Routes only and except work with routes too: # Whitelist routes resources :notes, only: [:show, :new, :create] # Blacklist routes resources :notes, except: [:index, :edit, :update, :destroy] 11 / 34

12 Associations 12 / 34

13 What does this do? # app/models/user.rb class User < ApplicationRecord has_many :tweets end # app/models/tweet.rb class Tweet < ApplicationRecord belongs_to :user, dependent: :destroy end 13 / 34

14 Associations So Far Last week we learned about belongs_to and has_many We talked about how this is represented in the model, but didn't spend a lot of time talking about at the database level 14 / 34

15 Keys Here are two kinds of keys in SQL databases: primary keys foreign keys There are others that we won't focus on in this class A key is essentially represented as a single column in a table. 15 / 34

16 Primary Keys Unique identifiers for each record Typically represented by an auto-incrementing id Our database tables can have one column for a primary key Active record just calls the column id 16 / 34

17 Foreign Keys Foreign keys are identifiers, which can help relate the record with a record in another table One record can have multiple foreign keys A foreign key will be a reference to a primary key of another table For example, the notes table would have a foreign key column notebook_id that references the id column in the notebooks table 17 / 34

18 Using Foreign Keys The foreign key will reference the id of the object it "belongs to" For example, an notebook with an id of 1 would have notes with a notebook_id of 1 You can insert new records into the database in relation to other objects and the foreign key will be automatically assigned 18 / 34

19 Using Foreign Keys notebook = Notebook.create(name: 'Notebook 1') p notebook.id #=> 1 note = notebook.notes.create(title: 'Note 1') p note.notebook_id #=> 1 note = Note.create(title: 'Note 2') notebook.notes << note p note.notebook_id #=> 1 19 / 34

20 Which table should get the foreign key column? Suppose we have an app that manages menus and their dishes. The menus table should have the foreign key column The dishes table should have the foreign key column Vote at PollEv.com/CIS / 34

21 Which model should have the belongs_to and which should have the has_many? Menu - has_many, Dish - belongs_to Menu - belongs_to, Dish - has_many Vote at PollEv.com/CIS / 34

22 Nested Routes It is often customary to use nested routes when a belongs_to, has_many relationship is defined Not always the case as you saw in the previous homework For example, if notes belong to notebooks, then you may only want to be able to access note routes through a specific notebook 22 / 34

23 Nested Resources # config/routes.rb resources :notebooks do resources :notes end Notebook routes will be defined as normal, but the routes for notes will look like this: Index: /notebooks/:notebook_id/notes Show: /notebooks/:notebook_id/notes/:id New: /notebooks/:notebook_id/notes/new etc. 23 / 34

24 has_many has_many But what if you wanted to create a has_many has_many relationship between models? For example, a note can have many editors and an editor can have many notes 24 / 34

25 has_many has_many But what if you wanted to create a has_many has_many relationship between models? For example, a note can have many editors and an editor can have many notes We can make an editor_id column in the notes table and a note_id in the editors table 24 / 34

26 Problems with that We can add the note to the editor's list of notes but then you also have to explicitly add the editor to the note's list of editors That's just redundant It also leads to duplication at the database level 25 / 34

27 Solution: SQL Joins There are several kinds of SQL joins (i.e. inner, left, right, and full), but we will only be using inner joins in this class An inner join looks for matches between two tables and returns all rows with matches Active Record typically matches one table's foreign key and another's primary key 26 / 34

28 Join Table The join table will contain foreign keys for the two tables you want to join It is convention to combine the two table names in alphabetical order e.g. The join table for notes and editors would be editors_notes So, for this note and editor example, you would have three tables: notes, editors, and editors_notes 27 / 34

29 How to represent Join Tables in Rails Use the has_many :through relationship 28 / 34

30 has_many :through To create the EditorsNotes join table: rails g migration CreateEditorsNotes editor:references note:references id editor_id note_id No editors_notes yet... # app/models/editors_notes.rb belongs_to :editor belongs_to :note 29 / 34

31 has_many :through # app/models/note.rb has_many :editors_notes, dependent: :destroy has_many :editors, through: :editors_notes # app/models/editor.rb has_many :editors_notes, dependent: :destroy has_many :notes, through: :editors_notes 30 / 34

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33 End Result The editor is automatically added to notes.editors editor = Editor.create note = editor.notes.create p note.editors.include? editor #=> true 32 / 34

34 Deleting You'll notice that we put the dependent: :destroy option on editors_notes In almost all instances, you want the join table relationship deleted as well It doesn't make sense for an editors_note to exist if the note no longer does We don't want to put the dependent: :destroy on the editor because editors can exist without a note 33 / 34

35 Homework 6 Penn InTouch Lite You'll be pulling together: nested routes associations (including has_many :through) before_action only and except Will be a 2 part HW Part 2 released next week, will involve building a front-end and deploying 34 / 34