從相關表，Rails獲取數據

Question

一些字段表城市

create_table "cities", force: :cascade do |t|
t.string "name"

一些字段表

create_table "orders", force: :cascade do |t|
t.integer "count"
t.integer "first_city_id"
t.integer "last_city_id"

模型訂單

class Order < ActiveRecord::Base
belongs_to :city, inverse_of: :city
end

模范城市

class City < ActiveRecord::Base
has_many :orders, inverse_of: :order
end

訂單視圖中的工作輸出數據

<% @order.each do |orders| %>
<p> <%= orders.count %> </p>
<% end %>

控制器中的方法

def edit
@orders= Order.all
@order=Order.where(:user_id == current_user)
end

如果Order表中的first_city_id或last_city_id包含城市表的ID必需條目，如何從名稱Sity表中獲取值？

Answer 1

您應按以下步驟設置訂單關聯：

class Order < ActiveRecord::Base
  belongs_to :first_city, class_name: "City" # it would use first_city_id as the foreign key by default to access city table
  belongs_to :last_city, class_name: "City"
end

從訂單中獲得城市的名字很容易：

@order = Order.first

@order.first_city.name
@order.last_city.name

如果要從訂單列表中獲得城市的名字/姓氏，則應使用includes或joins來避免n + 1查詢：

@orders = Order.includes(:first_city, :last_city)

<% @orders.each do |order| %>
  <p> First City: <%= order.first_city.name %> </p>
  <p> Last City: <%= order.last_city.name %> </p>
<% end %>

要么

@orders = Order.
  joins("LEFT JOIN cities AS first_cities ON first_cities.id = orders.first_city_id").
  joins("LEFT JOIN cities AS last_cities ON last_cities.id = orders.last_city_id").
  select("orders.*, first_cities.name AS first_city_name, last_cities.name AS last_city_name")

<% @orders.each do |order| %>
  <p> First City: <%= order.first_city_name %> </p>
  <p> Last City: <%= order.last_city_name %> </p>
<% end %>

您可以從這兩個文檔“ RailsGuides-Active Record Associations ”和“ RailsGuides-Active Record查詢接口 ”獲得更多信息和幫助。