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?

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

I think it might need something like:

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

More examples: ActiveRecord::Validations::ClassMethods

Or even more concise (with the new hash syntax), from the http://api.rubyonrails.org/classes/ActiveModel/Validations/ClassMethods.html#method-i-validates">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.

From the validates_length_of documentation:

validates_length_of :phone, :in => 7..32, :allow_blank => true

> :allow_blank - Attribute may be blank; skip validation.

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?}

validates_length_of :reason, minimum: 3, maximum: 30

rspec for the same is

it { should validate_length_of(:reason).is_at_least(3).is_at_most(30) }

How about that: validates_length_of :foo, is: 3, allow_blank: true

Add in your model:

validates :color, length: { is: 7 }

color is a string:

t.string :color, null: false, default: '#0093FF', limit: 7

In your model e.g.

def validate
  errors.add_to_base 'error message' unless self.foo.length == 5 or self.foo.blanc?
end