从Rails中的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

我有一个名为 Category 的模型，如下所示:

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

我有一个视图，其中显示了所有类别及其一些属性.我可以访问 category.parent_id，但我希望能够执行类似 category.parent_name 的操作.
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对应父名称填充集合，但我想知道是否有任何方法可以轻松做到这一点.

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.

我已经修改了模型，使其像这样:

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

创建表类别的迁移是这样的:

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

但是，当我将类别对象传递给视图时，我无法通过执行 category.parent.name 来访问其父属性 - 对该对象执行 inspect给我:

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">

如果我对 category.parent 进行检查，我会得到这个:

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">

但是，如果我尝试执行 category.parent.name，我会收到以下错误:

However if I try to do category.parent.name I get the following error:

undefined method `name' for nil:NilClass

我试图访问在我上面提到的对象之前为零的父级.这样做:

I was trying to access a parent that was nil before the object that I mentioned above. Doing this:

category.parent.try(:name) 

正如 Michael Irwin 在其中一个答案中所建议的那样解决了这个问题.

as suggested by Michael Irwin in one of the answers solved it.

推荐答案

自引用关联第一次很难...

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

然后你可以调用 category.children 和 category.parent 并访问关联对象的所有属性，...

Then you could call category.childrenand category.parent and also access all the attributes of the asscoiated oobjects,...

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