Model

Components

• ActiveRecord does the heavy lifting

• Key Idea: Models hold all the data & business logic

• Callback

  • before_save, before_create, before_update
  • after_save, after_create, after_update
• Relationships
  • has_many
  • belongs_to
• Scopes
  • scope :alphabetical, -> { order('last_name, first_name') }
• Validations: to ensure data integrity
  • validates_presence_of
  • validates_format_of
  • validates_inclusion_of
• Methods
  • privates methods - store callback code

Goals of Active Record

• Make working with databases easier
• Reduce repetition in code
• Cut down on configuration needed to make applications work

Primary Key and Active Record

• Active Record gives you a way of overriding the default name of the primary key for a table.

  • isbn = candidate key self.primary_key = “isbn”

• Table names are plural and class names are singular

  • pet model
    • belongs_to :owner
    • @pet.owner
  • OwnersController / owners table
    • has_many :pets
    • @owner.pets

Associations

• Def: a relationship or an association is a connection between two Active Record models
• ActiveRecord creates SQL
• Pet.last.owner 找到pet的最后一个的owner，相当于join table
• belong_to = have the foreign key
• Pet.last.visits - 须有一个table叫visits，然后里面有一个field叫pet_id

我们现在在pet.rb file里面 用.visit是因为有 belongs_to :visits

Pet.active.class => Pet::ActiveRecord_Relation 然后再输入Pet.active.alphabetical 就会return一个table

Pet.alphabetical.active 应该会fail，因为group by之后就不能go back to where 但是他行了。。it doesn't execute until the user press enter.

relationship on array 不行，会有error

eg. 一个owner有多个pets，有owner model 和 pet model - To add a new pet for an existing owner, we’d need to include the owner_id as a foreign key (in pet model). - When we’d like to delete owner having id=1, we should ensure that all his pets get deleted as well.

Active Record supports the three common types of relationship between tables: 1. one-to-one 2. one-to-many 3. many-to-many

Rails supports six types of associations: 1. belongs_to 2. has_many 3. has_many :through 4. has_one 5. has_one :through 6. has_and_belongs_to_many

Database

ORM

• Object-relational mapping (ORM): simplifies the use of databases in applications
• ActiveRecord: The Rails ORM

ActiveRecord Demo

• rails console
• Animal.all = select *
• Animal.first
• Animal.last
• Animal.find 3

Q: 这些method都是从哪里来的？ A: active record: the rails ORM

Q: Animal.find_by_name "Dog" 我们有这个method嘛？ A: 好在有meta programming 发生了某些神奇的东西，可以find by (field name) 就可以去找这个field

• dog = Animal.find(3) -- error
• dog.name
• dog.active

• 这些点后面的都是field name, return 这些field里面的值

• dog.name = 'Dog!!'

  • change in the memory
  • not change in the database

• dog.save / dog.save!

• dog.reload
  • save in the memory

Connecting to a database

Key 1: database.yml - in config/ directory - contains connection information for 3+ levels - development: db/development.sqlite3 - test - production - others possible, eg.staging - 今天全是讲的development level，之后还有test level，test完就到了production level

Key 2: Gemfile provides adapter - use sqlite3 in development & test - use Postgres or MySQL in production

• production就不用sqlite3（因为工业界不太在用）用oracle之类的
• sqlite3 -- 适合 embedded systems（一两百个record很适合，几百万个就不太适合）
• sqlite太慢了 比Postgres慢多了

Rails migrations

class CreatePets < ActiveRecord::Migration[5.2]
    def change
        create_table :pets do |t|
            t.string :name
            t.references :animal, foreign_key: true
            t.references :owner, foreign_key: true
            t.boolean :female
            t.date :date_of_birth
            t.boolean :active, default: true

            # t.timestamps
        end
    end
end

• migrations = instructions for how to build the database
  • create / change / drop tables
• Migrations are stored as files in the db/migrate directory, one for each migration class.
• File name: db/migrate/YYYYMMDDHHMMSS_create_proverbs.rb
• 不能rerun migration 不然会error; only run migrations that haven't run yet!!
  • 这个东西可能会考
• migration datatypes

Six operations of Rails Migration

1. add a new table (associated to a model)
   1. YYYYMMDDHHMMSScreatetable.rb

2. add a column

   1. rails generate migration AddColumnToTable column1:integer

   2. generates YYYYMMDDHHMMSS_add_column_to_table.rb

   3. rails generate migration AddDetailsToOwners age:integer ssn:string

   4. class AddDetailsToOwners < ActiveRecord::Migration[5.2] def change add_column :owners, :age, :integer add_column :onwers, :ssn, :string end end
   5. remove a column
   6. rails generate migration RemoveColumnToTable column:string
   7. drop a table
   8. rails destroy model ModelName
   9. add data to a table
   10. rake db:seed
   11. rake db:populate and rake db:contexts
   12. via migrations: create a default user at the time db is created

       class AddDefaultUser < ActiveRecord::Migration[5.2] 
           def change 
               # create a new instance of User and populate 
               vet = User.new 
               vet.username = "vet" 
               vet.email = "vet@example.com" 
               vet.password = "yodel" 
               vet.password_confirmation = "yodel" 
               # save with bang will throw exception on failure 
               vet.save! 
           end 
       end
       

   13. add functions and triggers to the database

       class AddPgSearchDmetaphoneSupportFunctions < ActiveRecord::Migration[5.2 ] 
           def change 
               # add the extension first... 
               say_with_time("Adding psql extensions if not existing:") do 
                   execute <<-'SQL' 
                   CREATE EXTENSION IF NOT EXISTS fuzzystrmatch; 
                   SQL 
           end 
       
           # now add the search function needed on the database side... 
           say_with_time("Adding support functions for pg_search :dmetaphone") do 
               execute <<-'SQL' 
               CREATE OR REPLACE FUNCTION pg_search_dmetaphone(text) 
                   RETURNS text LANGUAGE SQL IMMUTABLE STRICT AS $function$ 
                   SELECT array_to_string(ARRAY(SELECT dmetaphone(unnest(regexp_split_to_array($1, E'\\s+')))), ' ') 
                   $function$; 
               SQL 
               end 
           end 
       end