App Academy W5D4

Models, migrations & Associations

Rails: A server-side MVC web-application framework

• ****full stack framework*****

如何建立一个新的rails project 都可以

ActiveRecord - an ORM Framework (The M in MVC)

Allows us to:

• Represent models and their data
• Represent associations between data
• Validate models before they get persisted to the database
• Perform database operations (CRUD) in OOP fashion
• Migrations
• Models
• Associations

Migrations

• Incremental and reversible changes made to a database schema, allowing it to evolve over time.
• Not just a Rails thing - ubiquitous to app frameworks that work with relational DBs.
• Rails allows you to use an easy Ruby DSL (domain-specific language) to describe changes to your tables, rather than write raw SQL.

Let's Migrate!

Note:

Migration demo

rails g migration createUsers 大小写，复数 convention

t.string column_name,

add_index :users, :username, unique: true

add_index :users, :email, unique: true

2.

rails g migration AddAgeToUsers

AddAgeToUsers class

def change
   add_column :users, :age, :integer, null: false
end
db:migrate
check schema.rb
will see t.integer "age", null, false

bgg createChirps
class CreateChirps < ActiveRecord::Migration[5.2]
  def change
    create_table :chirps do |t|
      t.text :body, null: false
      # string <= 255, text longer
      t.integer :author_id, null: false #这里的null false都是数据库层面的constraint而不是rails本身的validation。
    end
    add_index :chirps, :author_id # cannot unique this one, can have a lot of authors in the table
  end
end
br db:migrate
tweets的表建立好了。

*****Common migration terminal commands*****

• bundle exec rails g migration Create{TableName} #必须是复数
• bundle exec rails g migration Add{ColumnName}To{TableName}
• bundle exec rails g migration Remove{ColumnName}From{TableName}
• bundle exec rails g migration AddIndexTo{TableName}

*****Common migration methods***************

• create_table
• add_column
• change_column
• remove_index

Changing existing migrations

• You can't just edit the migration and run the migration again
• Instead, you have two options:

1. Write a new migration (much preferred) migration就是用来改变数据库的工具，你在model里面修改再多的class都没用，那些都是调用数据库的函数而已，并没有对数据库进行真正意义的修改，只有是做了一个migration，这才是对数据库进行改进，比如说加个column，改个表名啊这些，这些操作一旦进行，有的就很难撤回，所以要非常小心，关于表格的操作还是要更仔细的看一看。addtable会了，但是drop table啊什么的这些还没见过。
2. Rollback: 这种操作其实非常不推荐，也不太现实，未来在真正的production环境下是不可能允许任何人rollback的，假如说某个数据库操作删掉了某个行，那就只能通过一些操作再把它加回来，rollback不是万金油，很多时候都不好用。

3. up down，在很多文档中看到过关于up和down的介绍，但是不知道怎么用，有时间要研究一下。

*****************************

• rollback the migration (via rails db:rollback)
• then edit your migration
• run rails db:migrate to run the corrected version

Model

• The central component of the MVC pattern
• A class that represents and directly manages the data, logic, and rules for a table 必须是单数存在的
• Typically contains: validations, associations, and custom methods
• Inherits from ApplicationRecord (which, in turn, inherits from ActiveRecord) 不直接继承active record的原因是不想修改它，多一层保护。
• There is a one-to-one correspondence between a model and a table，每个表对应一个class，每个instance对应一条数据。
• An instance of this class / model represents a row in our table

Database Constraints vs Model Validations

• Model validations are best used to provide error messages to users interacting with your app # validation更好，error更好看，初步防线，而model的contraints是最后防线
• It's highly likely that:
• You will interact with the database at some point outside of Rails
• You will make a mistake in your code that causes invalid data
• Database constraints are the last line of defense for data-integrity
• Writing constraints is work, but they will save you a lot of pain

Common Validations

• validates :some_column, presence: true
• similar to null: false
• validates :other_column, uniqueness: true
• similar to unique: true
• Custom Validations

Rails Models Demo

gem 'annotate'
能把schema里面的表关系以comment的形式放到相应的model里面去
create user的时候，id是postgres提供的。
rails c 已经load了所有的model

create a user

v.save！如果不成功会raise error
v.save 这个只会给你返回个true false

fakev.errors.full_messages #return array of full message
fakev.errors.messages #return as hash
Chirp.create(body:"msg",author_id: some_user.id)

增加自己的validation
def body_too_long
if body && body.length > 140
      errors[:body] << "is too long"
  end
end
validate的时候，要注意，是errors[:class_name] << 'error message' 最好写的相关一点，这个如果validate不同多，errors就会直接停止数据库操作，并返回写好的数据信息了。
不再是validates了，而是validate
******不要忘记这个部分。validate :body_too_long #后面也不用写什么true什么的

select user.id from users join likes on likes.liker_id = users.id join chirps on users.id = chirps.author_id

• Connections between two Active Record models.
• 两个model之间的关系了，has_many, belongs_to, has_one, has_many_through…
• Make common operations simpler and easier in your code.
• We don't have to write anymore SQL JOIN statements
• Simply methods that we can call

class Chirp < ApplicationRecord
  belongs_to :author, primary_key: :id, foreign_key: :author_id, class_name: :User
  validates :body, presence: true
  validates :author_id, presence: true
  validate :body_too_long
def body_too_long
    if body && body.length > 140
      errors[:body] << "is too long"
    end
  end
end

Example Association

class Strike < ApplicationRecord
  belongs_to :student,
  	primary_key: :id,
  	foreign_key: :student_id,
  	class_name: :Student
end
凡是这种belongs to的就说明这个表的级别要相对低一些，它存了别人的id，就说明有一种隶属关系，user可以有很多的transction，但是user表绝对不可能存那么多transaction id在里面，那样存储就会非常的费，而且没有意义，join之后正常就都出来了，但是transaction这种级别很低的表，就会存很多别人的id，users，product，shopper，market...因为它就是最granular的级别。数据库的建立有的时候就是要考虑，哪些东西是真正需要建立关系的，关系（id）存在哪个表里面，这些其实是费时间的。
有时间的话，尝试建立一个tlq的数据库吧。

• belongs_to is an ActiveRecord method that takes the following arguments

def belongs_to(:name, options = {})
end

Associations Code Demo

*****Associations Recap*****

class Chirp < ApplicationRecord
  #validations go here
  belongs_to :user,
    primary_key: :id,
    foreign_key: :author_id,
    class_name: :User
end

class User < ApplicationRecord
  #validations go here
  has_many :chirps,
    primary_key: :id,
    foreign_key: :author_id,
    class_name: :Chirp
end
为什么primary——key都一样？
暂时记住就是conversion吧
belongs to的foreign key同时也是has——one has——many的foreign key，都一样，虽然关系是相反的，我们就这样决定。

has_one

• easily confused with belongs_to
• only write them if you've already made the corresponding belongs_to
• 这个反正还是看id存在哪吧，id存的那个model就是belongs——to，否则就是has one

Strategy when writing associations

• Start with belongs_to
• Write the corresponding has_many or has_one in the other model.
• Write has_many throughs using only other associations in the model as the through, check the associated model for an association to be the source.
• has many through这个东西很牛逼，就是把表和表相连接了，association级别的join，model中互相就通过某个id，就连在一起。

Terminal Commands Recap

• rails new {project_name} -G -d postgresql or
• rails _VERSION_ new {project_name} -G -d postgresql
• bundle exec rails db:create
• bundle exec rails g migration Create{TableName}
• bundle exec rails db:migrate
• bundle exec rails db:migrate:status
• bundle exec rails g model {ModelName}