访问 has_one 关联的属性 [英] Accessing a has_one associations' attributes
问题描述
我对 Rails 还是很陌生，所以希望这不是一个愚蠢的问题.
I'm still quite new to Rails so hopefully this isn't a silly question.
我有两个模型:User 和 Chore.用户有_一个家务，家务属于用户
I have two models: User and Chore. User has_one chore, and Chore belongs to User
class User < ActiveRecord::Base
attr_accessible :chore_done, :email, :name, :phone
has_one :chore
class Chore < ActiveRecord::Base
attr_accessible :name, :user_id
belongs_to :user
在我的用户索引中，我试图显示所有用户并显示与他或她相关的琐事.我这样做是通过将 User.all 传递给视图并使用 .each 来遍历每个用户:
In my user index, I'm trying to show all users and display the chore that is associated with him or her. I'm doing this by passing User.all to the view and using a .each to iterate through each user:
<% @users.each do |user| %>
<tr>
<td><%= user.name %></td>
<td><%= user.chore.name %></td>
<td><%= user.chore_done? %></td>
<td><%= link_to 'Edit', edit_user_path(user) %></td>
<td><%= link_to 'Destroy', user, method: :delete, data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
很遗憾，我无法访问 Chore 的 name 属性.我收到此错误:
Unfortunately, I cannot access the name attribute of the Chore. I get this error:
nil:NilClass 的未定义方法 `name'
undefined method `name' for nil:NilClass
如果我删除 .name 属性，它只会返回一个指向 Chore 对象的指针.
If I remove the .name attribute, it just returns a pointer to the Chore object.
我有一种感觉，这与将 User.all 对象传递给视图然后对其进行迭代有关.只需在控制台中访问特定的用户对象(例如 User.find(1))，然后访问 user.chore.name 即可正常工作.
I have a feeling this has something to do with passing the User.all object to the view and then iterating over that. Just accessing a specific User object (e.g. User.find(1)) in the console, and then accessing user.chore.name works fine.
1.9.3-p194 :045 > user = User.find(4)
User Load (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1 [["id", 4]]
=> #<User id: 4, name: "Example User", email: "example@dot.com", phone: 8675309, chore_done: false, created_at: "2012-11-05 01:53:33", updated_at: "2012-11-05 01:53:33">
1.9.3-p194 :046 > user.chore.name
Chore Load (0.3ms) SELECT "chores".* FROM "chores" WHERE "chores"."user_id" = 4 LIMIT 1
=> "Living room"
我在 APIDock 中阅读了一些有关 AssociationProxy 的内容，但我认为我没有完全理解它，也不知道如何使用它.从我收集的信息来看，访问模型中的所有对象会返回一个对象数组，但不会为其依赖项返回一组完整的属性?因此，在这种情况下，我获得了指向 Chore 对象的指针，但无法访问其任何属性.
I read a bit about AssociationProxy in the APIDock, but I don't think I understand it completely nor do I know how to work with it. From what I gathered, accessing all objects in a model returns an Array of objects but doesn't return a complete set of attributes for its dependencies? So, in this case, I get a pointer to the Chore object but no access to any of its attributes.
我当然可以将 Chore 添加为我的用户表中的另一列，但我将来可能会添加到该模型中 + 我只想弄清楚 has_one 关联是如何工作的.
I could certainly just add Chore as another column in my user table, but I may add to that model in the future + I just want to figure out how the has_one association works anyway.
感谢任何帮助！
推荐答案
nil:NilClass 的未定义方法 `name'
undefined method `name' for nil:NilClass
这表示您正在尝试在 Nil 的实例上调用某个方法 name().在您的代码上下文中，该行的 chore 是 nil
This says that you are trying to call some method name() on an an instance of Nil. Within your code's context, that says that chore on this line is nil
<td><%= user.chore.name %></td>
简单地说，@user 中的 User 实例之一没有关联的 Chore.在您的视图中适应这一点的一种简单方法是检查 user.chore 是否存在.
Simply put, one of the User instances in @user has no associated Chore. One simple way to accomodate this in your view is by checking if user.chore exists.
<td><%= user.chore.nil? ? "some default name" : user.chore.name %></td>
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