Lecture 6. Active Record Associations 1 / 30

Transcription

1 Lecture 6 Active Record Associations 1 / 30

2 Homeworks 4 & 5 Homework 4 was graded Homework 5 was due last night Any questions? 2 / 30

3 Learn HTML Time's almost up! You're going to be writing your own views in Homework 7 3 / 30

4 Generators 4 / 30

5 More generators! Let's learn a couple of new generators! Controller Generator: rails g controller controller_name action1 action2 Scaffold Controller Generator: rails g scaffold_controller model_name col1:type col2:type 5 / 30

6 Controller Generator You pass the generator the name of the controller and the actions that you want inside of it Will generate the controller file with the actions that you pass along with it Also generates stubbed out view files for the actions It should be used when generating a controller that isn't associated with a model rails g controller welcome index 6 / 30

7 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 It will also generate the views for each of these actions It should be used when you want the 7 RESTful routes and a model is associated rails g scaffold_controller note notebook:references title:st 7 / 30

8 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: [:action_name, :other_action] # To call the method before all actions except a select few: before_action :method_name, except: [:action_name, :other_action] # Example: before_action :set_note, only: [:show, :edit, :update, :destroy] 8 / 30

9 Routes only and except work with routes too: # Will only generate show, new, and create routes for notes resources :notes, only: [:show, :new, :create] # Will only generate show, new, and create routes for notes resources :notes, except: [:index, :edit, :update, :destroy] 9 / 30

10 Associations 10 / 30

11 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 Let's talk a little bit more about this! 11 / 30

12 Keys Here are three kinds of keys in SQL databases: primary keys unique keys foreign keys There are others that we won't focus on in this class For our purposes, keys will be represented as single columns 12 / 30

13 Primary Keys Primary keys are unique identifiers for each record Primary keys are typically represented by an autoincrementing id Our database tables can have one column for a primary key Active record just calls it id 13 / 30

14 Unique Keys Unique keys are also unique identifiers for each record, but a table can have multiple unique keys A primary key is a type of unique key 14 / 30

15 Foreign Keys Foreign keys are identifiers, which can help relate the record with a record in another table A 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 15 / 30

16 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 16 / 30

17 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 17 / 30

18 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 18 / 30

19 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. 19 / 30

20 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 20 / 30

21 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 21 / 30

22 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 22 / 30

23 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 You can name the join table whatever you want since convention is a little weaker here, but this is the default 23 / 30

24 How to represent Join Tables in Rails There are different ways to do this, but we will use the has_many :through relationship 24 / 30

25 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 25 / 30

26 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 26 / 30

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28 End Result The editor is automatically added to notes.editors editor = Editor.create note = editor.notes.create p note.editors.include? editor #=> true 28 / 30

29 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 29 / 30

30 Homework 6 Penn InTouch Lite You'll be pulling together: nested routes associations (including has_many :through) before_action only and except 30 / 30