Ruby-on-rails rails 验证： :allow_nil 和 :inclusion 同时需要
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rails validation: :allow_nil and :inclusion both needed at the same time
提问by Daniel
Usually the field 'kind' should be allowed blank. but if it is not blank, the value should included in ['a', 'b']
通常应该允许字段“种类”为空。但如果它不为空，则该值应包含在 ['a', 'b'] 中
validates_inclusion_of :kind, :in => ['a', 'b'], :allow_nil => true
The code does not work?
代码不起作用？
回答by Matt
This syntax will perform inclusion validation while allowing nils:
此语法将在允许 nils 的同时执行包含验证：
validates :kind, :inclusion => { :in => ['a', 'b'] }, :allow_nil => true
回答by vasylmeister
In Rails 5 you can use allow_blank: trueoutside or inside inclusion block:
在 Rails 5 中，您可以使用allow_blank: true外部或内部包含块：
validates :kind, inclusion: { in: ['a', 'b'], allow_blank: true }
or
或者
validates :kind, inclusion: { in: ['a', 'b'] }, allow_blank: true
tip: you can use in: %w(a b)for text values
提示：您可以in: %w(a b)用于文本值
回答by Oren
check also :allow_blank => true
还检查：allow_blank => true
回答by Fabrizio Regini
If you are trying to achieve this in Rails 5 in a belongs_toassociation, consider that the default behaviour requires the value to exist.
如果您试图在 Rails 5 中以belongs_to关联方式实现这一点，请考虑默认行为要求该值存在。
To opt out from this behaviour you must specify the optionalflag:
要退出此行为，您必须指定optional标志：
belongs_to :foo, optional: true
validates :foo, inclusion: { in: ['foo', 'bar'], allow_blank: true }
回答by W.M.
In Rails 5.x you need, in addition to the following line, to call a before_validationmethod:
在 Rails 5.x 中，除了以下行之外，您还需要调用一个before_validation方法：
validates_inclusion_of :kind, :in => ['a', 'b'], :allow_nil => true
The before_validationis needed to convert the submitted blank value to nil, otherwise ''is not considered nil, like this:
将before_validation需要转换的提交空白值nil，否则''不认为nil，这样的：
before_validation(on: [:create, :update]) do
self.kind = nil if self.kind == ''
end
For database disk space usage it is of course better to store nil's than storing empty values as empty strings.
对于数据库磁盘空间的使用，存储nil's 比将空值存储为空字符串当然更好。
