ruby-on-rails - Ruby on Rails 让外键发挥作用

Question

我觉得这是一个我无法弄清楚的简单错误，但我没有在网上找到任何内容来帮助解释我做错了什么。我在 Ruby on Rails 应用程序中创建了三个表:

Facility
 facility_name - string
 address       - string
 city_id       - integer
 state_id      - integer
 
 and foreign keys: cities, states

City
  city_name    - integer
  state_id     - integer

State
  state_short  - string
  state_long   - string

我将模型定义为:

class Facility < ApplicationRecord
  has_one :city
  has_one :state
end


class City < ApplicationRecord
  belongs_to :state
  belongs_to :facility
end

class State < ApplicationRecord
  has_many :cities
  belongs_to :facility
end

我认为我可以通过 Rails 控制台上的以下代码在设施内显示城市和州:

x = Facility.first
x.city_name
x.state_long

但是这两个代码都会出错。我是否错误地认为，通过在设施表中包含 city_id 和 state_id 以及上述关系，我应该能够获取 city_name和 state_long 来自各自的表？

Answer 1

class Facility < ApplicationRecord
  has_one :city
  has_one :state
end

这意味着cities和states表有facility_id列

但是在您的架构中设施具有city_id和state_id

这意味着你需要以这种方式改变你的关联

class Facility < ApplicationRecord
  belongs_to :city
  belongs_to :state
end

class City < ApplicationRecord
  belongs_to :state
  has_one :facility
end

class State < ApplicationRecord
  has_many :cities
  has_one :facility
end

另一个选项是更改架构(如果保留关联)

facilities
-----------
facility_name
address

cities
-----------
city_name
state_id 
facility_id

states
-----------
state_short
state_long
facility_id

换句话说你need choose在 belongs_to 和 has_one

之间

The distinction is in where you place the foreign key (it goes on the table for the class declaring the belongs_to association), but you should give some thought to the actual meaning of the data as well. The has_one relationship says that one of something is yours - that is, that something points back to you

在任何情况下，如果设施实例没有 city_name 和 state_long 方法，都会发生错误，因为添加关联不会定义关联模型的方法

你可以这样调用它

facility = Facility.first
facility.city.city_name
facility.state.state_long

但是可以添加这样的方法

class Facility < ApplicationRecord
  belongs_to :city
  belongs_to :state

  def city_name
    city.city_name
  end

  def state_long
    state.state_long
  end
end

或使用委托(delegate)方法

class Facility < ApplicationRecord
  belongs_to :city
  belongs_to :state

  delegate :city_name, to: :city
  delegate :state_long, to: :state
end

之后你可以这样调用它

facility = Facility.first
facility.city_name
facility.state_long