Rails:如何在 ActiveRecord 中设置默认值? [英] Rails: How can I set default values in ActiveRecord?

问题描述

如何在 ActiveRecord 中设置默认值?

我看到 Pratik 的一篇文章描述了一段丑陋而复杂的代码:http://m.onkey.org/2007/7/24/how-to-set-default-values-in-your-model

class Item  解决方案 

每种可用方法都有几个问题，但我相信定义 after_initialize 回调是解决问题的方法以下原因:

1. default_scope 将初始化新模型的值，但这将成为您找到模型的范围.如果您只想将一些数字初始化为 0，那么这不是您想要的.
2. 在您的迁移中定义默认值在某些时候也有效...正如已经提到的那样，当您只调用 Model.new 时，这将起作用.
3. 覆盖initialize 可以工作，但不要忘记调用super！
4. 使用像 phusion 这样的插件有点荒谬.这是 ruby​​，我们真的需要一个插件来初始化一些默认值吗?
5. 覆盖 after_initialize 从 Rails 3 开始被弃用.当我在 Rails 3.0.3 中覆盖 after_initialize 时，我在控制台中收到以下警告:

<块引用>

弃用警告:Base#after_initialize 已弃用，请改用 Base.after_initialize :method.(从/Users/me/myapp/app/models/my_model:15 调用)

因此我想说写一个 after_initialize 回调，它允许你默认属性除了让你像这样设置关联的默认值:

 class Person 

现在您只有一个地方可以查找模型的初始化.我一直在使用这种方法，直到有人想出更好的方法.

注意事项:

1. 对于布尔字段:

   self.bool_field = true if self.bool_field.nil?

   有关详细信息，请参阅 Paul Russell 对此答案的评论

2. 如果您只是为模型选择列的子集(即；在像 Person.select(:firstname, :lastname).all 这样的查询中使用 select) 如果您的 init 方法访问未包含在 select 子句中的列，您将得到 MissingAttributeError.您可以像这样防范这种情况:

   self.number ||= 0.0 如果 self.has_attribute?:number

   对于布尔列...

   self.bool_field = true if (self.has_attribute? :bool_value) &&self.bool_field.nil?

   另请注意，Rails 3.2 之前的语法有所不同(请参阅下面的 Cliff Darling 评论)

How can I set default value in ActiveRecord?

I see a post from Pratik that describes an ugly, complicated chunk of code: http://m.onkey.org/2007/7/24/how-to-set-default-values-in-your-model

class Item < ActiveRecord::Base  
  def initialize_with_defaults(attrs = nil, &block)
    initialize_without_defaults(attrs) do
      setter = lambda { |key, value| self.send("#{key.to_s}=", value) unless
        !attrs.nil? && attrs.keys.map(&:to_s).include?(key.to_s) }
      setter.call('scheduler_type', 'hotseat')
      yield self if block_given?
    end
  end
  alias_method_chain :initialize, :defaults
end

I have seen the following examples googling around:

  def initialize 
    super
    self.status = ACTIVE unless self.status
  end

and

  def after_initialize 
    return unless new_record?
    self.status = ACTIVE
  end

I've also seen people put it in their migration, but I'd rather see it defined in the model code.

Is there a canonical way to set default value for fields in ActiveRecord model?

解决方案

There are several issues with each of the available methods, but I believe that defining an after_initialize callback is the way to go for the following reasons:

1. default_scope will initialize values for new models, but then that will become the scope on which you find the model. If you just want to initialize some numbers to 0 then this is not what you want.
2. Defining defaults in your migration also works part of the time... As has already been mentioned this will not work when you just call Model.new.
3. Overriding initialize can work, but don't forget to call super!
4. Using a plugin like phusion's is getting a bit ridiculous. This is ruby, do we really need a plugin just to initialize some default values?
5. Overriding after_initialize is deprecated as of Rails 3. When I override after_initialize in rails 3.0.3 I get the following warning in the console:

DEPRECATION WARNING: Base#after_initialize has been deprecated, please use Base.after_initialize :method instead. (called from /Users/me/myapp/app/models/my_model:15)

Therefore I'd say write an after_initialize callback, which lets you default attributes in addition to letting you set defaults on associations like so:

  class Person < ActiveRecord::Base
    has_one :address
    after_initialize :init

    def init
      self.number  ||= 0.0           #will set the default value only if it's nil
      self.address ||= build_address #let's you set a default association
    end
  end    

Now you have just one place to look for initialization of your models. I'm using this method until someone comes up with a better one.

Caveats:

1. For boolean fields do:

   self.bool_field = true if self.bool_field.nil?

   See Paul Russell's comment on this answer for more details

2. If you're only selecting a subset of columns for a model (ie; using select in a query like Person.select(:firstname, :lastname).all) you will get a MissingAttributeError if your init method accesses a column that hasn't been included in the select clause. You can guard against this case like so:

   self.number ||= 0.0 if self.has_attribute? :number

   and for a boolean column...

   self.bool_field = true if (self.has_attribute? :bool_value) && self.bool_field.nil?

   Also note that the syntax is different prior to Rails 3.2 (see Cliff Darling's comment below)

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