rails validation: :allow_nil and :inclusion both needed at the same time

Usually the field 'kind' should be allowed blank. but if it is not blank, the value should included in ['a', 'b']

validates_inclusion_of :kind, :in => ['a', 'b'], :allow_nil => true

The code does not work?

This syntax will perform inclusion validation while allowing nils:

validates :kind, :inclusion => { :in => ['a', 'b'] }, :allow_nil => true

In Rails 5 you can use allow_blank: true outside or inside inclusion block:

validates :kind, inclusion: { in: ['a', 'b'], allow_blank: true }

validates :kind, inclusion: { in: ['a', 'b'] }, allow_blank: true

tip: you can use in: %w(a b) for text values

check also :allow_blank => true

If you are trying to achieve this in Rails 5 in a belongs_to association, consider that the default behaviour requires the value to exist.

To opt out from this behaviour you must specify the optional flag:

belongs_to :foo, optional: true 

validates :foo, inclusion: { in: ['foo', 'bar'], allow_blank: true }

In Rails 5.x you need, in addition to the following line, to call a before_validation method:

validates_inclusion_of :kind, :in => ['a', 'b'], :allow_nil => true

The before_validation is needed to convert the submitted blank value to nil, otherwise '' is not considered nil, like this:

  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.

来源：https://stackoverflow.com/questions/17359853/rails-validation-allow-nil-and-inclusion-both-needed-at-the-same-time

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