Question

我有一种情况，我希望在保存父对象之前访问相关的祖父母。我可以想到几个黑客，但我正在寻找一种干净的方法来实现这一目标。以下代码为例说明我的问题：

class Company < ActiveRecord::Base
  has_many :departments
  has_many :custom_fields
  has_many :employees, :through => :departments
end
class Department < ActiveRecord::Base
  belongs_to :company
  has_many :employees
end
class Employee < ActiveRecord::Base
  belongs_to :department
  delegate :company, :to => :department
end

company = Company.find(1)           # => <Company id: 1>
dept = company.departments.build    # => <Department id: nil, company_id: 1>
empl = dept.employees.build         # => <Employee id: nil, department_id: nil>

empl.company   # => Employee#company delegated to department.company, but department is nil

我正在使用Rails 3.2.15。我理解这里发生了什么，我理解为什么empl.department_id是零;虽然我希望Rails在调用save之前直接引用预期关联，这样最后一行可以通过未保存的部门对象委派。有干净的工作吗？

更新：我在Rails 4中也试过这个，这是一个控制台会话：

2.0.0-p247 :001 > company = Company.find(1)
  Company Load (1.5ms)  SELECT "companies".* FROM "companies" WHERE "companies"."id" = ? LIMIT 1  [["id", 1]]
 => #<Company id: 1, name: nil, created_at: "2013-10-24 03:36:11", updated_at: "2013-10-24 03:36:11"> 
2.0.0-p247 :002 > dept = company.departments.build
 => #<Department id: nil, name: nil, company_id: 1, created_at: nil, updated_at: nil> 
2.0.0-p247 :003 > empl = dept.employees.build
 => #<Employee id: nil, name: nil, department_id: nil, created_at: nil, updated_at: nil> 
2.0.0-p247 :004 > empl.company
RuntimeError: Employee#company delegated to department.company, but department is nil: #<Employee id: nil, name: nil, department_id: nil, created_at: nil, updated_at: nil>
2.0.0-p247 :005 > empl.department
 => nil

更新2：这是test project on github。

Answer 1

请查看belongs_to和has_many的:inverse_of选项。在不同情况下构建和获取关联记录时，此选项将处理双向分配。

来自文档中ActiveRecord::Associations::ClassMethods的{{3}}：

在关联上指定:inverse_of选项可让您告诉Active Record有关反向关系的信息，它将优化对象加载。

Answer 2

我不喜欢这个解决方案，但这似乎解决了这个问题：

empl = dept.employees.build { |e| e.association(:department).target = dept}

事实证明，您可以传递一个块来构建，ActiveRecord将使用新创建的记录生成块。谁知道ActiveRecord会带来什么怪异。我现在打开这个问题，看看是否有更好的解决方案。

Answer 3

经过一些控制台实验 - 您可以说employee.department.company，即使部门尚未保存。 department_id可能为零，但department关联就在那里。

2.0.0-p195 :041 > c = Company.create 
   (0.4ms)  begin transaction
  SQL (0.9ms)  INSERT INTO "companies" DEFAULT VALUES
   (486.4ms)  commit transaction
 => #<Company id: 4, department: nil, custom_fields: nil> 
2.0.0-p195 :042 > d = c.departments.build
 => #<Department id: nil, company_id: 4, employee_id: nil> 
2.0.0-p195 :043 > e = d.employees.build
 => #<Employee id: nil, department_id: nil> 
2.0.0-p195 :044 > e.department === d
 => true 
2.0.0-p195 :045 > e.department.company === c
 => true

编辑：所以，这不适用于另一台干净的Rails 4应用程序。但是，它仍然适用于我的笔记本电脑...也在一个干净的Rails 4应用程序。让我们试着弄清楚有什么不同！

e.method(:department)
=> #<Method: Employee(Employee::GeneratedFeatureMethods)#department> 

e.method(:department).source_location
=> ["/home/neil/.rvm/gems/ruby-2.0.0-p195/gems/activerecord-  
    4.0.0/lib/active_record/associations/builder/association.rb", 69]

这引导我们：

def define_readers
  mixin.class_eval <<-CODE, __FILE__, __LINE__ + 1
    def #{name}(*args)
      association(:#{name}).reader(*args)
    end
  CODE
end

确实没有惊喜，这定义了一个名为:department

的方法

def department *args
  association(:department).reader(*args)
end

对reader的调用只返回关联的@target（如果它存在），或者如果它有一个id，则尝试读取它。在我的情况下，@target设置为部门d。要发现设置@target的点，我们可以拦截target=中的ActiveRecord::Associations::Association：

class ActiveRecord::Associations::Association 
  alias :_target= :target=
  def target= t
    puts "#{caller} set the target!"
    _target = t
  end
end

现在，当我们致电d.employees.build时，我们得到了这个......

"/home/neil/.rvm/gems/ruby-2.0.0-p195/gems/activerecord-4.0.0/lib/active_record/associations/association.rb:112:in `set_inverse_instance'", 
"/home/neil/.rvm/gems/ruby-2.0.0-p195/gems/activerecord-4.0.0/lib/active_record/associations/collection_association.rb:376:in `add_to_target'",
"/home/neil/.rvm/gems/ruby-2.0.0-p195/gems/activerecord-4.0.0/lib/active_record/associations/collection_association.rb:114:in `build'"

set_inverse_instance正在检查invertible_for?(record)，（其中record是我们的新员工实例。）这只是调用reflection.inverse_of，这必须返回一个真值，以便要设定的目标。

def inverse_of
  return unless inverse_name

  @inverse_of ||= klass.reflect_on_association inverse_name
end

所以让我们尝试一下......

2.0.0-p195 :055 > Employee.reflect_on_association :department
 => #<ActiveRecord::Reflection::AssociationReflection:0xa881788 @macro=:belongs_to, @name=:department, @scope=nil, @options={}, @active_record=Employee(id: integer, department_id: integer), @plural_name="departments", @collection=false, @class_name="Department", @foreign_key="department_id">

那是非零的，所以当我拨打d.employee.build时，@ target会在我的关联中设置，所以我可以拨打e.department，依此类推。那么为什么它在这里是非零的，但对你来说是零（在我的另一台机器上呢？）如果我打电话给Employee.reflections，我会得到以下结果：

> Employee.reflections
 => {:department=>#<ActiveRecord::Reflection::AssociationReflection:0x9a04598 @macro=:belongs_to, @name=:department, @scope=nil, @options={}, @active_record=Employee(id: integer, department_id: integer), @plural_name="departments", @collection=false, @class_name="Department", @foreign_key="department_id">}

这是belongs_to方法的产物 - 如果你看，它必须在那里。那么为什么（在你的情况下）不set_inverse_instance找到它？

如何通过尚未保存的父关联访问ActiveRecord祖父母关联？

3 个答案: