从相关表，Rails获取数据

Question

some fields tables City

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

some fields tables Order

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

model Order

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

model City

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

working output data from Order in view

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

Method in controller

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

How to get the value from the name Sity table if first_city_id or last_city_id in the Order table contains id required entry of City Table?

Answer 1

You should setup the order associations as follows:

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

Getting the first/last city name from an order becomes easy:

@order = Order.first

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

If you want to get the first/last city name from an order list, you should use includes or joins to avoid the n+1 query:

@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 %>

or

@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 %>

You can get more information and helps from these two docs " RailsGuides - Active Record Associations " and " RailsGuides - Active Record Query Interface ".