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

Question

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?

Answer 1

This syntax will perform inclusion validation while allowing nils:

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