1:N 一對多

如果我們希望每間商店可以販售許多種飲料，可以使用一對多模式來進行關連。

先建立 Drink 這個 Model：
rails g model Drink name price:decimal store_id:integer

目前的資料關聯設計像下圖

先把 model 之間的關聯建立好

drink.rb

class Drink < ApplicationRecord
  belongs_to :stores
end

store.rb

class Store < ApplicationRecord
  has_many :drinks
  belongs_to :owner
end

方法建立

跟 has_one 一樣，設定 has_many :drinks 後會多了以下幾個方法，belongs_to 建立的方法跟1:1是一樣，這邊就不多提：

drinks
drinks=
build
create

console 測試看看

# 找回上次建立的飲料店
s1 = Store.last

id: 1,                                 
name: "五石蘭",                        
tel: "031234567",                      
address: "新竹縣",                     
owner_id: 1,                           
created_at: Fri, 07 Oct 2022 07:22:23.353390000 UTC +00:00,
updated_at: Fri, 07 Oct 2022 07:22:23.353390000 UTC +00:00> 

# 直接用 Drink 建立好再丟給 store
d1 = Drink.new(name:"紅茶", price:30)

id: 1,                                                                         
name: "紅茶",                                                              
price: 0.6e2,                                                                  
store_id: 1,                                                                   
created_at: Fri, 07 Oct 2022 07:52:50.417919000 UTC +00:00,
updated_at: Fri, 07 Oct 2022 07:52:50.417919000 UTC +00:00>,

#1 把 d1 放到陣列中指定給 s1.drinks
s1.drinks = [d1]

#2 把 d1 插入到 s1.drinks 陣列
s1.drinks << d1

# 透過 store 去建立 drink
s1.drinks.create(name:"珍珠奶茶", price:60)
                                
id: 2,                                                                         
name: "珍珠奶茶",                                                              
price: 0.6e2,                                                                  
store_id: 1,                                                                   
created_at: Fri, 07 Oct 2022 07:54:55.118310000 UTC +00:00,
updated_at: Fri, 07 Oct 2022 07:54:55.118310000 UTC +00:00>] 

查出所有s1的飲料

s1.drinks
 
id: 1,                                                  
name: "紅茶",                                           
price: 0.3e2,                                           
store_id: 1,                                            
created_at: Fri, 07 Oct 2022 07:52:50.417919000 UTC +00:00,
updated_at: Fri, 07 Oct 2022 07:52:50.417919000 UTC +00:00>,
id: 2,                                                  
name: "珍珠奶茶",                                       
price: 0.6e2,                                           
store_id: 1,                                            
created_at: Fri, 07 Oct 2022 07:54:55.118310000 UTC +00:00,
updated_at: Fri, 07 Oct 2022 07:54:55.118310000 UTC +00:00>] 

也可以反查出某款飲料是哪間店

# 找回最後建立的飲料(珍珠奶茶)
d2 = Drink.last

id: 2,                                                  
name: "珍珠奶茶",                                       
price: 0.6e2,                                           
store_id: 1,                                            
created_at: Fri, 07 Oct 2022 07:54:55.118310000 UTC +00:00,
updated_at: Fri, 07 Oct 2022 07:54:55.118310000 UTC +00:00>] 

# 因為 belongs_to 可以利用 store 方法反查
d2.store
 id: 1,                                                   
 name: "五石蘭",                                          
 tel: "031234567",                                        
 address: "新竹縣",                                       
 owner_id: 1,                                             
 created_at: Fri, 07 Oct 2022 07:22:23.353390000 UTC +00:00,
 updated_at: Fri, 07 Oct 2022 07:22:23.353390000 UTC +00:00> 

參考資料：

1. 為你自己學 Ruby on Rails