Ruby-on-rails 在 Rails 中添加可为空的外键
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Add nullable foreign key in Rails
提问by maicher
Referencing to Rails 4.2 add_foreign_keysupport:
参考 Rails 4.2 add_foreign_key支持：
# add a foreign key to `articles.author_id` referencing `authors.id`
add_foreign_key :articles, :authors
How to create a nullable foreign key constraint, to allow the situation, where articles.author_idcan be sometimes null?
如何创建一个可为空的外键约束，在允许的情况下，articles.author_id有时哪里可以为空？
采纳答案by user1322092
There is nothing in the guide that implies add_foreign_keywould make the corresponding foreign field "NOT NULL" or required. add_foreign_keysimply adds a foreign key constraint whether the field is required or not (in your case author_idin articles).
指南中没有任何内容暗示add_foreign_key会使相应的外部字段“NOT NULL”或必需。add_foreign_key只是添加字段是否需要与否（在你的情况下，外键约束author_id中articles）。
Did you get an error when you tried this in your migration?
您在迁移中尝试此操作时是否遇到错误？
Here's the SQL that it would generate:
这是它将生成的 SQL：
ALTER TABLE "articles" ADD CONSTRAINT articles_author_id_fk FOREIGN KEY ("author_id") REFERENCES "authors" ("id")
SO, if in your original migration of articles, author_idis null, then you can have foreign key that's nullable.
因此，如果在您的原始迁移中articles,author_id为空，那么您可以拥有可以为空的外键。
回答by Matthias Winkelmann
Note that in Rails 5and in Rails 6you may need to mark the corresponding association as optional if it's 1:n (belongs_to), as the default was changed:
请注意，在 Rails 5和Rails 6中，如果它是 1:n ( belongs_to)，您可能需要将相应的关联标记为可选，因为默认值已更改：
belongs_to :author, optional: true
belongs_to :author, optional: true
This is the corresponding Changeset.
这是相应的Changeset。
To use the old behavior across your application, you can also set:
要在您的应用程序中使用旧行为，您还可以设置：
Rails.application.config.active_record.belongs_to_required_by_default = false
in config/initializers/new_framework_defaults.rb
在 config/initializers/new_framework_defaults.rb
The error you will usually see is:
您通常会看到的错误是：
ActiveRecord::RecordInvalid: Validation failed: Class must exist
from /usr/local/lib/ruby/gems/2.3.0/gems/activerecord-5.0.0.1/lib/active_record/validations.rb:78:in `raise_validation_error'
回答by Aarvy
Adding optional: truealong with belongs_to :authorin articlemodel will do the job.
optional: true与belongs_to :authorinarticle模型一起添加将完成这项工作。
