allow_blank_allow_nil.rb

# I have two models: `AllowBlank` and `AllowNil`:

class AllowBlank < ActiveRecord::Base
  validates :name, :presence => true, :allow_blank => true
end

class AllowNil < ActiveRecord::Base
  validates :name, :presence => true, :allow_nil => true
end

# The `AllowBlank` validations should always pass and the `AllowNil` validations
# should pass on `nil` values, but fail on empty strings (`''`).

# Let's try it:

irb(main):001:0> AllowBlank.new.valid?
=> false
irb(main):002:0> AllowBlank.new(:name => '').valid?
=> false
irb(main):003:0> AllowNil.new.valid?
=> false
irb(main):004:0> AllowNil.new(:name => '').valid?
=> false

# Sad panda. :(

# So, using `allow_blank` should probably not be supported at all, but `allow_nil` should fail on empty strings. Right? ;)