我如何在ActiveRecord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 < 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
我在谷歌上看到了以下例子:
def initialize
super
self.status = ACTIVE unless self.status
end
and
def after_initialize
return unless new_record?
self.status = ACTIVE
end
我也见过有人把它放在迁移中，但我更愿意看到它在模型代码中定义。
是否有一个规范的方法来设置默认值的字段在ActiveRecord模型?
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)
在开发Rails 6应用程序时，我也遇到过类似的挑战。
以下是我的解决方法:
我有一个用户表和一个角色表。Users表属于Roles表。我也有一个管理员和学生模型，从用户表继承。
然后需要在创建用户时为角色设置一个默认值，比如id = 1的管理员角色或id = 2的学生角色。
class User::Admin < User
before_save :default_values
def default_values
# set role_id to '1' except if role_id is not empty
return self.role_id = '1' unless role_id.nil?
end
end
这意味着在数据库中创建/保存admin用户之前，如果role_id不为空，则将其设置为默认值1。
return self.role_id = '1' unless role_id.nil?
等于:
return self.role_id = '1' unless self.role_id.nil?
和:
self.role_id = '1' if role_id.nil?
但是第一种方法更简洁，也更精确。
这是所有。
我希望这对你们有帮助
