在rails 3中使用default_scope

接口文档

ActiveRecord模糊了在数据库(模式)中定义的默认值和在应用程序(模型)中完成的默认值之间的区别。在初始化过程中，它解析数据库模式并记录其中指定的任何默认值。稍后，在创建对象时，它将分配那些模式指定的默认值，而不涉及数据库。

讨论

Phusion有一些很好的插件。

这就是构造函数的作用!重写模型的initialize方法。

使用after_initialize方法。

我已经用了一段时间了。

# post.rb
class Post < ApplicationRecord
  attribute :country, :string, default: 'ID'
end

after_initialize解决方案的问题是，无论是否访问该属性，都必须向从DB中查找的每个对象添加after_initialize。我建议采用惰性加载方法。

属性方法(getter)当然是方法本身，因此您可以覆盖它们并提供默认值。喜欢的东西:

Class Foo < ActiveRecord::Base
  # has a DB column/field atttribute called 'status'
  def status
    (val = read_attribute(:status)).nil? ? 'ACTIVE' : val
  end
end

除非，就像有人指出的那样，你需要执行Foo.find_by_status('ACTIVE')。在这种情况下，我认为您确实需要在数据库约束中设置默认值，如果数据库支持它的话。

每个可用的方法都有几个问题，但我认为定义一个after_initialize回调是正确的方法，原因如下:

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. 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. Overriding initialize can work, but don't forget to call super! 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? 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:

弃用警告:Base#after_initialize已弃用，请使用Base。After_initialize:方法代替。(调用from /Users/me/myapp/app/models/my_model:15)

因此，我要说的是写一个after_initialize回调函数，它允许你在关联上设置默认属性，就像这样:

  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    

现在您只有一个地方可以寻找模型的初始化。我一直在用这个方法，直到有人提出更好的方法。

警告:

For boolean fields do: self.bool_field = true if self.bool_field.nil? See Paul Russell's comment on this answer for more details 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)