在Rails中联接两个表 [英] Joining two tables in rails

问题描述

我正在尝试链接位置和操作表，以便可以在操作视图的位置表中显示一些数据。但是，我被困住了，不知道该怎么办。任何帮助，将不胜感激。

I am trying to link the location and the operating tables so that I can display some data in location table in the operatings views. However, I am stuck and don't know what to do. Any help would be appreciated.

 #below are models#
 class Location < ApplicationRecord  
    has_many :operatings
 end

 class Operating < ApplicationRecord
    belongs_to :location
 end

 ##below are my tables##

 enable_extension "plpgsql"

 create_table "locations", force: :cascade do |t|
    t.string   "country"
    t.string   "supra_region"
    t.string   "region"
    t.datetime "created_at",   null: false
    t.datetime "updated_at",   null: false
 end

 create_table "operatings", force: :cascade do |t|
  t.string   "operating_company_name"
  t.string   "address"
  t.date     "year_formed"
  t.string   "other_operational_countries"
  t.string   "about_company"
  t.string   "current_focus"
  t.string   "incumbent_irm_contractor"
  t.string   "irm_frame_agreements"
  t.text     "estimated_irm_budgets"
  t.integer  "location_id"
  t.datetime "created_at",                  null: false
  t.datetime "updated_at",                  null: false
  t.index ["location_id"], name: "index_operatings_on_location_id", using: :btree
 end

 add_foreign_key "operatings", "locations"

 ###below is my operating controller###

  def create
     @operating = Operating.new(op_company)
     if @operating.save
        flash[:success] = "A recorded has been successfully Saved"
        redirect_to operatings_path
     else
        render 'new'
     end
  end

 ####routes####
 resources :offshores,    :index, :show, :new, :create, :destroy
 resources :locations,    :index, :show, :new, :create, :destroy

推荐答案

因为您的位置和 Operating 模型使用 has_many 和 belongs_to ，如果模板中有 operating 对象，则可以轻松访问其位置的属性：

Since your Location and Operating models are linked together using has_many and belongs_to, if you have an operating object in your template, you can easily access the attributes of its location:

<% @operatings.each do |operating| %>
  <div>The name of its location: <%= operating.location.name %></div>
<% end %>

不过，您需要对此小心。如果仅从数据库中获取操作，则在每个每个循环中访问每个操作的 location 属性将触发一个单独的数据库查询每个操作项目。这称为 N + 1 查询，效率非常低。要解决此问题，在使用 includes 加载操作时，请确保预取相关的位置：

You need to be careful with this though. If you only fetch the operatings from the database, accessing each operating's location attribute in that each loop will trigger a separate database query for every operating item. This is called an N+1 query, and it is very inefficient. To fix the problem, make sure to pre-fetch the associated location as well when loading operatings using includes:

# in the controller
@operatings = Operating.all.includes(:location)

这样，每个操作的关联位置将仅通过单个查询获取。

This way the associated locations of every operating will be fetched using just a single query.

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