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

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 using the below validations in the model, but this doesn't allow the user to leave it blank:

validates_length_of :foo, :maximum => 5
validates_length_of :foo, :minimum => 5

How do I write this to accomplish my goal?

Solution 1:

You can also use this format:

validates :foo, length: {minimum: 5, maximum: 5}, allow_blank: true

Or since your min and max are the same, the following will also work:

validates :foo, length: {is: 5}, allow_blank: true

Solution 2:

I think it might need something like:

validates_length_of :foo, minimum: 5, maximum: 5, allow_blank: true

More examples: ActiveRecord::Validations::ClassMethods

Solution 3:

Or even more concise (with the new hash syntax), from the validates documentation:

validates :foo, length: 5..5, allow_blank: true

The upper limit should probably represent somehting more meaningful like "in: 5..20", but just answering the question to the letter.