在Rails中从子对象访问父对象属性 [英] Accessing parent object attribute from child's object in Rails

问题描述

  class Category< p> ActiveRecord :: Base 
 has_many：categories 
 belongs_to：category，：foreign_key => parent_id
 end 
  

我有一个视图，它显示了所有的类别属性。我可以访问 category.parent_id ，但我希望能够做一些像 category.parent_name 。 >
我可以看到自己创建了一个模型方法来获取所有类别，并用每个类别的记录父类名称来填充集合，但是我想知道是否有这样的方法。

编辑：我修改了模型，使其具有这样的效果：

  class Category< ActiveRecord :: Base 
 has_many：children，：class_name => 'Category'，：foreign_key => 'parent_id'
 belongs_to：parent，：class_name => 'Category'，：foreign_key => 'parent_id'
 end 
  

创建表类别的迁移过程如下所示： p>

  class CreateCategories< ActiveRecord :: Migration 
 def change 
 create_table：categories do | t | 
 t.string：name 
 t.text：description 
 t.integer：parent_id 
 
 t.timestamps 
 end 
 end 
结束
  

但是，当我传递一个类别对象到一个视图，我无法访问其父通过执行 category.parent.name - 做一个 inspect 这个对象给我：

 < Category id：2，name：Test 2，description：Prova 2，parent_id：1，created_at：2012-01- 17 19:28:33，updated_at：2012-01-17 19:28:33> 
  

如果我检查 category.parent

 ＃< Category id：1，name：Prova，description：Test， parent_id：nil，created_at：2012-01-17 19:28:17，updated_at：2012-01-17 19:28:17> 
  

但是如果我试着做 category.parent.name



 未定义方法`name'为nil：NilClass 
  

编辑2：我试图访问父对象，我上面提到的零。这样做：



  category.parent.try（：name）
  由 Michael Irwin 中的一个答案解决了它。  
 
自引用关联在第一次很难...  
 
 
  class类别< ActiveRecord :: Base 
 has_many：children，：class_name => 'Category'，：foreign_key => 'parent_id'
 belongs_to：parent，：class_name => 'Category'，：foreign_key => 'parent_id'
 end 
  
然后你可以调用 category.children 和 category.parent ，并且还可以访问asscoiated对象的所有属性，... 
 
I have a model called Category which looks like this:
class Category < ActiveRecord::Base
  has_many :categories
  belongs_to :category,:foreign_key => "parent_id"
end
I have a view which shows all the categories with some of their attributes.  I can access category.parent_id, but I would like to be able to do something like category.parent_name.

I can see myself creating a model method to fetch all categories and filling the collection with the correspondent parent name of each category, but I'm wondering if there is anyway to do this easily.  

EDIT: I have modified the model to have it like this:
class Category < ActiveRecord::Base
  has_many :children, :class_name => 'Category', :foreign_key => 'parent_id'
  belongs_to :parent, :class_name => 'Category', :foreign_key => 'parent_id'
end
The migration to create the table categories is like this:
class CreateCategories < ActiveRecord::Migration
  def change
    create_table :categories do |t|
      t.string :name
      t.text :description
      t.integer :parent_id

      t.timestamps
    end
  end
end
However when I pass a category object to a view I am not able to access its parent attributes by doing category.parent.name - Doing an inspect of that object gives me:
<Category id: 2, name: "Test 2", description: "Prova 2", parent_id: 1, created_at: "2012-01-17 19:28:33", updated_at: "2012-01-17 19:28:33">
And if I do an inspect of category.parent I get this:
#<Category id: 1, name: "Prova", description: "Test", parent_id: nil, created_at: "2012-01-17 19:28:17", updated_at: "2012-01-17 19:28:17">
However if I try to do category.parent.name I get the following error:
undefined method `name' for nil:NilClass
EDIT2: I was trying to access a parent that was nil before the object that I mentioned above.  Doing this:
category.parent.try(:name) 
as suggested by Michael Irwin in one of the answers solved it.
解决方案
Self referencing associations are hard at the first time...
class Category < ActiveRecord::Base
  has_many :children, :class_name => 'Category', :foreign_key => 'parent_id'
  belongs_to :parent, :class_name => 'Category', :foreign_key => 'parent_id'
end
Then you could call category.childrenand category.parent and also access all the attributes of the asscoiated oobjects,...

                        
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