如何为 MySQL/Rails 中的列获取随机默认值 [英] How do I get a random default value for a column in MySQL/Rails

问题描述

假设我有一个 mysql 表，其中包含诸如#EE3AA"之类的颜色值.而且我有一个具有颜色属性的模型，该属性是强制性的(应该验证)，但不一定需要由用户输入.如果用户留空，我希望使用自动从另一个表中选择的随机值填充它，但默认情况下并不总是相同的值.

Let's say I have a mysql table full of color values like "#EE3AA". And I have a model which has a color attribute, which is obligatory (should validate) but does not need to be entered by the user necessarily. If left empty by the user, I want it to be filled using a random value chosen from another table automatically, but not always the same value by default.

我想，也许，我可以使用迁移来完成，所以它是这样的:

I thought, perhaps, I could do it using a migration, so it is like this:

... :default => "values of the color column of the Colors table".to_a.sample

我的想法在 Ruby 方面可能不正确/不合法.在迁移文件中看到 :default 选项后，我认为每次创建模型时都会运行迁移文件.但阅读米哈伊尔的评论，情况可能并非如此.但无论如何，当用户没有提供任何值时，我如何从另一个表中获取随机值?

My thinking may not be correct/legal in terms of Ruby. After seeing the :default option in the migrations files, I thought that the migration file is run every time a model is created. But reading Mikhail's comment, this may not be the case. But in any case, how I can get a random value from another table when none is supplied by the user?

推荐答案

迁移仅在您创建数据库或更改数据库结构时运行，而不是在保存新记录时运行.Mikhail 建议使用 before_save 回调是正确的.你可以在你的模型中做这样的事情:

Migrations are only run when you create a database or make changes to the structure of the database, not when saving a new record. Mikhail is correct in suggesting a before_save callback. You can do something like this in your model:

class Model < ActiveRecord::Base
  before_save :set_empty_color_to_random_value

  def set_empty_color_to_random_value
    self.color = Color.order("RANDOM()").first.color if self.color.empty?
  end
end

请注意，RANDOM() 适用于 PostgreSQL 和 SQLite，但不适用于 MySQL.对于 MySQL，您必须使用 RAND() 代替.

Note that RANDOM() works for PostgreSQL and SQLite, but not for MySQL. For MySQL you have to use RAND() instead.

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