Question

我正在学习Ruby on Rails，我制作了一个非常简单的应用程序。这是schema.rb：

ActiveRecord::Schema.define(version: 20160112141616) do

  create_table "cities", force: :cascade do |t|
    t.string   "name"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer  "state_id",   null: false
  end

  add_index "cities", ["state_id"], name: "index_cities_on_state_id"

  create_table "states", force: :cascade do |t|
    t.string   "name"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

end

我装了一些州和城市。而且我不知道如何使用Active Record进行这样的查询：

SELECT cities.name, states.name FROM cities INNER JOIN states ON cities.state_id = states.id

我想这样做是为了在一个视图中打印，所有的城市名称都带有州名。

感谢。

修改

model city.rb：

class City < ActiveRecord::Base
  belongs_to :states
  validates :state_id, presence: true
end

model state.rb：

class State < ActiveRecord::Base
  has_many :cities
end

cities_controller.rb：

class CitiesController < ApplicationController
  def index
    #@cities = City.all.order(state_id: :asc)
    #@cities = State.joins(:cities).select('cities.name, states.name')
    @cities = City.joins(:states).select('cities.name, states.name')
  end

  def new
    @city = City.new
    @states = State.all.order(name: :asc)
  end

  def create
    @city = City.new(city_params)
    if @city.save(:validate=> true)
      redirect_to :back
    else
      render :new
    end
  end

  def city_params
    params.require(:city).permit(:name, :state_id)
  end
end

Answer 1

以下查询将执行

City.joins(:states).select("cities.name, states.name")

NameError：未初始化的常量City :: States

belongs_to的 关联名称 应为单数，因此请更改{{1在states模型中的state

city.rb

从两个表中查询活动记录

1 个答案: