Difference between the presence and allow_blank validators in Rails?

Question

I'm trying to figure out the difference between:

validates :foo, presence: false
validates :foo, allow_blank: true

When I use presence: false validation fails but when I use allow_blank: true it does not. According to the docs http://apidock.com/rails/ActiveRecord/Validations/ClassMethods/validates_presence_of uses the blank? method. Can someone please explain the difference?

Answer 1

First case:

validates :foo, presence: false

it does not validate the presence of :foo at all.

nil, '', 'anything' are all valid.

Second case: :allow_blank is an option, not a validator.

It skips validation if the attribute is blank (more here).

If you want to know how it works, you can see the code here.

Before call the selected validator, it checks the attribute is not blank, if it's then skip validation.

The only reason why it works as a validator is the way that source code is written.

At any moment Rails' team can change the code and :allow_blank stop working as a validator.

Answer 2

allow_blank only validates nil, presence validates nil as well as empty