Rails唯一性约束和空列的匹配数据库唯一索引 [英] Rails uniqueness constraint and matching db unique index for null column

问题描述

我的迁移文件中包含以下内容

I have the following in my migration file

  def self.up
    create_table :payment_agreements do |t|
      t.boolean    :automatic, :default => true, :null => false
      t.string     :payment_trigger_on_order
      t.references :supplier
      t.references :seller
      t.references :product
      t.timestamps
    end
  end

我想确保如果指定了product_id，那么它是唯一的，但我也想允许null，因此我的模型中包含以下内容:

I want to ensure that if a product_id is specified it is unique but I also want to allow null so I have the following in my model:

  validates :product_id,
            :uniqueness => true,
            :allow_nil => true

效果很好，但是我应该在迁移文件中添加一个索引

Works great but I should then add an index to the migration file

add_index :payment_agreements, :product_id, :unique => true

很显然，当为product_id插入两个null值时，这将引发异常.我可以简单地在迁移中省略索引，但是有机会获得具有相同product_id的两个PaymentAgreements，如下所示:

Obviously this will throw an exception when two null values are inserted for product_id. I could just simply omit the index in the migration but then there's the chance that I'll get two PaymentAgreements with the same product_id as shown here: Concurrency and integrity

我的问题是解决这个问题的最好/最常见的方法是什么

My question is what is the best/most common way to deal with this problem

推荐答案

这取决于您的数据库服务器. 至于mysql:

it depends on your db server. as for mysql:

UNIQUE索引创建约束 这样索引中的所有值都必须 与众不同.如果您发生错误 尝试添加具有键值的新行 匹配现有行.这 约束不适用于NULL 值(BDB存储除外) 引擎.对于其他引擎，唯一 索引允许多个NULL值用于 可以包含NULL的列.

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does not apply to NULL values except for the BDB storage engine. For other engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL.

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