Ruby-on-rails 在 rails 中使用带有 has_one 关联的构建
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Using build with a has_one association in rails
提问by espinet
In this example, I create a userwith no profile, then later on create a profilefor that user. I tried using build with a has_oneassociation but that blew up. The only way I see this working is using has_many. The useris supposed to only have at most one profile.
在这个例子中，我创建了一个userno profile，然后profile为该用户创建了一个。我尝试使用带有has_one关联的构建，但失败了。我看到这个工作的唯一方法是使用has_many. 本user应该只有最多只能有一个profile。
I have been trying this. I have:
我一直在尝试这个。我有：
class User < ActiveRecord::Base
has_one :profile
end
class Profile < ActiveRecord::Base
belongs_to :user
end
But when I do:
但是当我这样做时：
user.build_profile
I get the error:
我收到错误：
ActiveRecord::StatementInvalid: Mysql::Error: Unknown column 'profiles.user_id' in 'where clause': SELECT * FROM `profiles` WHERE (`profiles`.user_id = 4) LIMIT 1
Is there a way in rails to have 0 or 1 association?
rails 有没有办法让 0 或 1 关联？
回答by Harish Shetty
The buildmethod signature is different for has_oneand has_manyassociations.
和关联的build方法签名不同 。has_onehas_many
class User < ActiveRecord::Base
has_one :profile
has_many :messages
end
The build syntax for has_manyassociation:
has_many关联的构建语法：
user.messages.build
The build syntax for has_oneassociation:
has_one关联的构建语法：
user.build_profile # this will work
user.profile.build # this will throw error
Read the has_oneassociation documentationfor more details.
阅读has_one关联文档以获取更多详细信息。
回答by sosborn
Take a good look at the error message. It is telling you that you do not have required column user_idin the profile table.
Setting the relationships in the model is only part of the answer.
仔细看看错误信息。它告诉您user_id在配置文件表中没有 required 列。在模型中设置关系只是答案的一部分。
You also need to create a migration that adds the user_idcolumn to the profile table.
Rails expects this to be there and if it is not you cannot access the profile.
您还需要创建一个将user_id列添加到配置文件表的迁移。Rails 希望它在那里，如果没有，您将无法访问该配置文件。
For more information please take a look at this link:
有关更多信息，请查看此链接：
回答by Shiyason
Depending on the use case, it can be convenient to wrap the method and automatically build the association when not found.
根据用例，可以方便地包装方法并在未找到时自动构建关联。
old_profile = instance_method(:profile)
define_method(:profile) do
old_profile.bind(self).call || build_profile
end
now calling the #profilemethod will either return the associated profile or build a new instance.
现在调用该#profile方法将返回关联的配置文件或构建一个新实例。
source: When monkey patching a method, can you call the overridden method from the new implementation?
回答by Karl
It should be a has_one. If buildisn't working, you can just use new:
它应该是一个has_one. 如果build不起作用，您可以使用new：
ModelName.new( :owner => @owner )
is the same as
是相同的
@owner.model_names.build
