使用抽象和界面(Use of abstract and interface)

 我是对的，您希望每个StudentForm属于一个 City吗？ 只有这样，您才能将city_id存储在StudentForm 。 但那么你的模型将是： 
 
class StudentForm < ActiceRecord::Base
  belongs_to :city

  delegate :name, to: :city. prefix: true
end

class City < ActiceRecord::Base
  has_many :student_forms
end
 
 你可以访问该城市的表格名称 
 
form = StudentForm.first
puts form.city.name
 
 要么 
 
puts form.city_name
 
 在您的示例中，每个StudentForm has_many :cities ，因此City需要对StudentForm的引用（列student_form_id ）。 

Am I right, that you want each StudentForm to belong to one City? Only then, you can store city_id in StudentForm. But then your models would be:
 
class StudentForm < ActiceRecord::Base
  belongs_to :city

  delegate :name, to: :city. prefix: true
end

class City < ActiceRecord::Base
  has_many :student_forms
end
 
and you could access the city's name of a form with
 
form = StudentForm.first
puts form.city.name
 
or
 
puts form.city_name
 
In your example, each StudentForm has_many :cities, so City needs a reference to StudentForm (a column student_form_id).