Rails 5 +

你可以在你的模型中使用属性方法，例如:

class Account < ApplicationRecord
  attribute :locale, :string, default: 'en'
end

您还可以将lambda传递给默认参数。例子:

attribute :uuid, :string, default: -> { SecureRandom.uuid }

第二个参数是类型，它也可以是一个自定义类型类实例，例如:

attribute :uuid, UuidType.new, default: -> { SecureRandom.uuid }

Phusion有一些很好的插件。

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)

Rails 5 +

你可以在你的模型中使用属性方法，例如:

class Account < ApplicationRecord
  attribute :locale, :string, default: 'en'
end

您还可以将lambda传递给默认参数。例子:

attribute :uuid, :string, default: -> { SecureRandom.uuid }

第二个参数是类型，它也可以是一个自定义类型类实例，例如:

attribute :uuid, UuidType.new, default: -> { SecureRandom.uuid }

这就是构造函数的作用!重写模型的initialize方法。

使用after_initialize方法。

class Item < ActiveRecord::Base
  def status
    self[:status] or ACTIVE
  end

  before_save{ self.status ||= ACTIVE }
end