Rails: Validating min and max length of a string but allowing it to be blank

Question

I have a field that I would like to validate. I want the field to be able to be left blank, but if a user is entering data I want it to be in a certain format. Currently I am us

Answer 1

every validates_* accepts :if or :unless options

validates_length_of :foo, :maximum => 5, :if => :validate_foo_condition

where validate_foo_condition is method that returns true or false

you can also pass a Proc object:

validates_length_of :foo, :maximum => 5, :unless => Proc.new {|object| object.foo.blank?}