Problem Records When Learning Ruby on Rails

Ruby on Rails 5 Essential Training: https://www.lynda.com/Ruby-Rails-tutorials/Ruby-Rails-5-Essential-Training/500551-2.html

Notice: Record every problem when i learned ruby on rails. I hope my experience can help you to learn ruby on rails better.

1. Get Started

(1) Create a project

(a) Use "brew install sql" to install sql

(b) Initialize ruby on rails project: rails new simple_cms -d mysql.

name is simple_cms, use mysql database

(2) Configure a project

(a) Input "mysql -u root -p" and the password of my mbp doestn't word.

Use "mysql.server start" to start the server of mysql;
The default password of the root is blank, just type "Enter" will be OK.

(b) Alter the database to other user.

Use "GRANT ALL PRIVILEGES ON simple_cms_test.* TO 'rails_user'@'localhost' IDENTIFIED BY 'secretpassword';";
simple_cms_test is the name of database, rails_user is the name of user,
localhost is the server of the user, secretpassword is the password of this database

(c) After changing the username and password in database.yml, using "rails db:schema:dump" to connect to the data base and dump to the text, get "rails aborted! Mysql2::Error: Access denied for user 'rails_user'@'localhost' ..."

Made mistake when setting the password in database.yml file, set the password as "secretspassword", the password set in the mysql is "secretpassword". Foolish mistake.

(3) Access a Project

(a) How to start and stop server.

Start: rails s
Stop: control + c

(4) Generate a controller and view

(a) What does "rails generate controller demo index" do.

Create a controller named 'demo' with an action named 'index'(the name of views).
'demo' is the folder name, 'index' is view template name.
Use "localhost:3000/demo/index" can check this view in the controller

(5) Server request handling

(a) Build folder named "demo" in public folder and add a file named "static_page.html"

web server first looks for a matching file on the public folder.
in browser, input "localhost:3000/demo/static_page.html" can visit this page

(6) Routes

(a) How to set default route?

get ':controller(/:action(/:id))'

(b) How to set default route?

root 'demo#index', tells which controller and action should route to.

2. Controllers, Views, and Dynamic Content

(1) Render a template

(a) How to use render to override the default route

In app/controllers/demo_controller.rb, use:
def index
	render('hello')
end
so when you visit /demo/index, you will view the template of "hello.html.erb".
render(:template => 'demo/hello')

(2) Redirect actions

(a) How to redirect to other pages?

It need the combination of config/routes.rb and apps/controllers/demo_controller.rb.
in routes.rb:
	def other_hello
		redirect_to(:action => 'hello')
	end
in demo_controller.rb
	get 'demo/other_hello'
the routes parse the address of browser and get other_hello string and send it to controller to handle it.

(3) Instance variable

(a) How to use instance variable.

In demo_controller.rb, define an instance variable like this followed by @,
def hello
	@array = [1,2,3,4,5]
	render('hello')
end
In hello.html.erb, we can use this array like:
<% @array.each do |n|>

(4) Link

(a) How to create a link

the link in ruby on rails like this: <%= link_to(text, target) %>
<%= link_to("one link", {:action => 'index'}) %>

(5) URL parameters

(a) How to use parameters in URL?

aa) In index.html.erb, construct a url link with parameters:
<%= link_to('Hello with parameters', {:action => 'hello', :page => 5, :id => 20}) %><br />
bb) In demo_controller.rb, fetch the parameters in url:
@id = params[:id]
@page = params[:page]
these instance variables can be used in templates
cc) In hello.html.erb, display this paramters:
ID: <%= @id %>
ID: <%= params[:id] %>

3. Datebase and migration

(1) Generate migrations

(a) the line to generate a migration.

rails generate migration DoNothingYet

(2) Generate models

(a) How to generate a model and add column in table.

aa) rails generate model User,
	get an xxx_create_users.rb in db/migrate
bb) t.column "first_name", :string, :limit => 25

(b) The relationship between database and rails

database = application
table = model
column = attribute of model

(3) Run migration

(a) Run "rails db:migrate" fail and get error.

Forget to start mysql server.

(b) Basic operation of mysql.

aa) rails -u rails_user -p simple_cms_development
bb) SHOW TABLES;
cc) SHOW FIELDS FROM users;

(c) How to down/up the migration

down: rails db:migrate VERSION=0;
use 'rails db:migrate:status' to find the version you want to up;
up: rails db:migrate VERSION=...;
schema file will keep track the version

4. Models and ActiveRecord

（1）Mdoel naming

(a) what is the relationship between files, classes and tables.

These names should be sync, for example:
file name: admin_user.rb
classe name: class AdminUser < ApplicationRecord
table name: admin_users

(2) The Rails console

(a) How to use rails console.

Use 'rails console' to access rails console.
Then we can use activerecord of the table, eg:
subject = Subject.new
subject.name = 'test'

(3) Create record

(a) difference between new and create.

aa) subject = Subject.new(:name => "First Subject", position: 1, visible: true, created_at: nil, updated_at: nil)
it does not save automatically
bb) subject = Subject.create(:name => "Second Subject", :position => 2)
it save the record automatically

(4) Update record

(a) two ways of updating record

aa) subject = Subject.find(1)
	subject.name = "Initial Subject"
	subject.save
it does not save automatically
bb) subject = Subject.find(2)
	subject.update_attributes(:name => "Next Subject", :visible => true)
it save the record automatically

(5) Delete record

(a) How to delete a record

subject = Subject.find(3)
subject.destroy
This record will be deleted from database

(6) Find records

(a) Three ways to find records

aa) subject = Subject.find(1)
bb) subject = Subject.find_by_name("Initial Subject")
cc) subjects = Subject.all

(7) Query methods: Condidtions

(a) Three ways to construct query by using where.

aa) subject = Subject.where(:visible => true)
bb) subject = Subject.where("visible = true")
cc) subject = Subject.where(["visible = ?", true])

(8) Query methods: Order, limit, and offset

(a) Different ways to use these methods.

aa) subjects = Subject.order(:position)
bb) subjects = Subject.order("position ASC")
cc) subjects = Subject.limit(1).offset(2)

(9) Named scopes

(a) How to use named scope?

In app/models/subject.rb, we can define some named scopes, these variable can be used in rails console. eg:
scope :visible, lambda { where(:visible => true) }
scope :invisible, lambda { where(:visible => false) }
scope :sorted, lambda { order("position ASC") }
scope :newest_first, lambda { order("created_at DESC") }
scope :serach, lambda { |query| where(["name LIKE ?", "%#{query}%"])}
in rails console, we can use like this:
Subject.invisible

5. Associations

(1)One-to-one associations

(a) How to build one-to-one associations.

in subjec.rb: has_one :page
in page.rb: belongs_to :subject
subject = Subject.find(1)
first_page = Page.new(...)
subject.page = first_page

(b) How to cancel one-to-one associations.

subject.page = nil
subject.page.destroy

(2) One-to-Many Associations

(a) How to build one-to-many associations.

	in subject.rb: has_many :pages
	in page.rb: belongs_to :subject
	subject.pages << first_page
	subject.pages << second_page

(b) How to cancel one-to-many associations.

	subject.pages.delete(second_page)

(3) Many-to-many associations: Simple

(a) How to build many-to-many associations

aa) Create a join table
generate migration CreateAdminUsersPagesJoin
in /dbmigrate file: 
	class CreateAdminUsersPagesJoin < ActiveRecord::Migration[5.1]
	  def up
	    create_table :admin_users_pages, :id => false do |t|
	      t.integer "admin_user_id"
	      t.integer "page_id"
	    end
	    add_index("admin_users_pages", ["admin_user_id", "page_id"])
	  end

	  def down
	    drop_table :admin_users_pages
	  end

	end
the name of the page is admin_users_pages
bb) set relationship
in admin_user.rb: has_and_belongs_to_many :pages
in page.rb: has_and_belongs_to_many :admin_users
cc) build relationship
me.pages << first_page

(4) Many-to-many associations: Rich

(a) How to build many-to-many associations use another table(model).

aa) rails generate model SectionEdit
bb) in db/migrate file:
	class CreateSectionEdits < ActiveRecord::Migration[5.1]

	  def up
	    create_table :section_edits do |t|
	      t.integer "admin_user_id"
	      t.integer "section_id"
	      t.string "summary"
	      t.timestamps
	    end
	    add_index("section_edits", ['admin_user_id', 'section_id'])
	  end

	  def down
	    drop_table :section_edits
	  end

	end
cc) rails db:migrate
dd) in admin_user.rb:
	has_many :section_edits
	in section.rb:
	has_many :section_edits
	in section_edit.rb
	belongs_to :admin_user
	belongs_to :section
ee) edit = SectionEdit.new(:summary => "test edit", :admin_user => me, :section => section)

(5) Traverse a rich association

(a) How to traverse a rich association

by using through
aa) in admin_user.rb: has_many :sections, :through => :section_edits
bb) in section.rb: has_many :admin_users, :through => :section_edits
cc) me.sections

6. CRUD, REST, and Resourceful Routes

(1) CRUD

(a) Create a controller with CRUD action.
rails generate controller Subjects index show new edit delete
aa) create a controller named SubjectController
	in this file, it added some actions:
	def index
	end
	...
bb) create a view folder named subjects
	this folder contains this correspoding views such as delete.html.erb ...
cc) create corresponding routes in routes.rb
	such as: get 'subjects/delete'

(2) REST

(a) REST HTTP Verbs

index - get
show - get
new - get
create - post
edit - get
update - patch
delete - get
destroy - delete

(3) Resourceful routes

(a) How to define and show resourceful routs

delete get 'subjects/delete' and add code:
resources :subject do
	member do
		get :delete
	end
end
this block of code add delete action

7. Controllers adn CRUD

(1) Read action: Index

(a) All the parts of MVC architecture.

aa) in url use /subjects
bb) Rails took the url and determine to use resourceful routes SubjectsController#index
cc) in subjects_controller, in index action, it operate some operations, the default render it to 'index.html.erb'
dd) in 'index.html.erb' view, show corresponding info
ee) the view sends back to the browser

(2) Read action: Show

(a) How to use helper to implement subject resourceful routes.

aa) index.html.erb
	<%= link_to("Show", subject_path(subject), :class => 'action show') %>
	or
	<%= link_to("Show", {:controller => "subjects", action => "show", :id => "subject.id"}, :class => 'action show') %>
bb) subjects_controller.rb
	def show
		@subject = Subject.find(params[:id])
	end
	use instance variable and get parameter from url
cc) show.html.erb
	use @subject to use the instance variable