给定以下的结构，其中一个 `Step` 拥有多个 `Items`，而一个 `Item` 拥有多个嵌套的 `Items`：
    class Step
      has_many :step_items
      has_many :items, through: :step_items
    end
    class Item
      has_one :step_item
      has_one :step, through: :step_item
      has_many :nested_items, foreign_key: :parent_item_id
      has_many :items, through: :nested_items
    end
    class StepItem
      belongs_to :step
      belongs_to :item
    end
    class NestedItem
      belongs_to :parent_item, foreign_key: :parent_item_id
      belongs_to :item
    end
在构建嵌套的 `Items` 时，如何访问关联模型呢？
例如，对于与 `Step` 相关的 `Item`，可以像这样检索 `StepItem`：
```ruby
step.items.build.step_item # => StepItem

item = step.items.build.items.build # => Item
item.??? # 如何检索连接 `item` 与其父级的 `NestedItem`？

请注意，这里的 "???" 表示您需要填入相应的代码来检索 `NestedItem` 模型。
<details>
<summary>英文:</summary>
Given the following structure where a `Step` has many `Items` and an `Item` has many nested `Items`:
    class Step
      has_many :step_items
      has_many :items, through: :step_items
    end
    class Item
      has_one :step_item
      has_one :step, through: :step_item
      has_many :nested_items, foreign_key: :parent_item_id
      has_many :items, through: :nested_items
    end
    class StepItem
      belongs_to :step
      belongs_to :item
    end
    class NestedItem
      belongs_to :parent_item, foreign_key: :parent_item_id
      belongs_to :item
    end
How could I access the join model when &quot;building&quot; nested `Items`? 
For example, for an `Item` that was related to a `Step` I could retrieve the `StepItem` like this:

How would I access the specific instance of the `NestedItem` model when building items:

</details>
# 答案1
**得分**: 1
Your terminology is a little off, so I'm not quite sure what you're asking:
例如，对于与步骤相关的项目，我可以这样检索StepItem：
step.items.build.step_item # => StepItem
这不是检索项目，而是创建一个新项目。但我认为你想要的是：
parent_item = step.items.build
item = parent_item.items.build # => Item
parent_item.nested_items.find{|ne| ne.item == item}
但你可能真正想要的是从项目到parent_item
展开收缩
的关系。
真正的问题可能是为什么你想要访问nested_item，因为它已经完成了它的整个工作。
<details>
<summary>英文:</summary>
Your terminology is a little off, so I&#39;m not quite sure what you&#39;re asking:

That's not retrieving an item, that's building a new one.  But I *think* what you want is

But you may really want a relationship from Item to parent_item
展开收缩
.
The real question may be why you want to get at the nested_item at all since it has done its whole job already.
</details>