代码之家 › 专栏 › 技术社区 › Daniel Abrahamsson

如何用多态性对has-u-many进行建模?

polymorphic-associations inheritance ruby-on-rails

Daniel Abrahamsson · 技术社区 · 16 年前

我遇到了一个不太确定如何建模的情况。

编辑:下面的代码现在表示一个有效的解决方案。不过,我还是对好看的解决方案感兴趣。

假设我有一个用户类,一个用户有许多服务。然而,这些服务是完全不同的,例如 MailService 和一个 BackupService

class User < ActiveRecord::Base
    has_many :services
end

class Service < ActiveRecord::Base
    validates_presence_of :user_id, :implementation_id, :implementation_type
    validates_uniqueness_of :user_id, :scope => :implementation_type

    belongs_to :user
    belongs_to :implementation, :polymorphic => true, :dependent => :destroy
    delegate :common_service_method, :name, :to => :implementation
end

#Base class for service implementations
    class ServiceImplementation < ActiveRecord::Base
    validates_presence_of :user_id, :on => :create

    #Virtual attribute, allows us to create service implementations in one step
    attr_accessor :user_id
    has_one :service, :as => :implementation

    after_create :create_service_record

    #Tell Rails this class does not use a table.
    def self.abstract_class?
        true
    end

    #Name of the service.
    def name
        self.class.name
    end

    #Returns the user this service
    #implementation belongs to.
    def user
        unless service.nil? 
            service.user
        else #Service not yet created
            @my_user ||= User.find(user_id) rescue nil
        end
    end

    #Sets the user this 
    #implementation belongs to.
    def user=(usr)
        @my_user = usr
        user_id = usr.id
    end

    protected

    #Sets up a service object after object creation.
    def create_service_record
        service = Service.new(:user_id => user_id)
        service.implementation = self
        service.save!
    end
end
class MailService < ServiceImplementation
   #validations, etc...
   def common_service_method
     puts "MailService implementation of common service method"
   end
end

#Example usage
MailService.create(..., :user => user)
BackupService.create(...., :user => user)

user.services.each do |s|
    puts "#{user.name} is using #{s.name}"
end #Daniel is using MailService, Daniel is using BackupService

那么,这是最好的解决方案吗?甚至是一个好的?你是怎么解决这种问题的?

2 回复 | 直到 16 年前

Dave Sims 16 年前

我认为你目前的解决办法行不通。如果ServiceImplementation是抽象的,那么关联的类将指向什么?如果ServiceImplementation没有将pk持久化到数据库中,那么另一端是如何工作的?也许我错过了什么。

编辑:哎呀,我原来的也不行。但这个想法仍然存在。代替一个模块,继续使用带有STI的服务,而不是多态性,并用单独的实现来扩展它。我认为您被STI和跨不同实现的一堆未使用的列困住了,或者重新思考了一般的服务关系。您拥有的委托解决方案可能作为一个单独的ActiveRecord工作,但是如果它必须有一个has\u one关系,我不认为它是如何作为抽象的工作的。

编辑:那么为什么不保留delgates而不是原来的抽象解决方案呢?对于MailServiceDelegate和BackupServiceDelegate,您必须有单独的表——如果您想用纯STI避免所有的空列,就不知道如何绕过这些表。您可以使用带有delgate类的模块来捕获常见关系和验证等。很抱歉,我花了几次时间才解决了您的问题:

class User < ActiveRecord::Base
  has_many :services
end

class Service < ActiveRecord::Base
  validates_presence_of :user_id
  belongs_to :user
  belongs_to :service_delegate, :polymorphic => true
  delegate :common_service_method, :name, :to => :service_delegate
end

class MailServiceDelegate < ActiveRecord::Base
   include ServiceDelegate

   def name
     # implement
   end

   def common_service_method
      # implement
   end
end

class BackupServiceDelegate < ActiveRecord::Base
   include ServiceDelegate

   def name
     # implement
   end

   def common_service_method
      # implement
   end
end

module ServiceDelegate
   def self.included(base)
     base.has_one :service, :as => service_delegate
   end

   def name
     raise "Not Implemented"
   end

   def common_service_method
     raise "Not Implemented"
   end
end

Salil 16 年前

我想下面的办法行得通

  has_many :mail_service, :class_name => 'Service'
  has_many :backup_service, :class_name => 'Service'

在用.rb

  belongs_to :mail_user, :class_name => 'User', :foreign_key => 'user_id', :conditions=> is_mail=true
  belongs_to :backup_user, :class_name => 'User', :foreign_key => 'user_id', :conditions=> is_mail=false