:allow_nil only working in Console?

Hello there,

I have the following Book model, where users can enter the Title of
the book and optionally the format of the ISBN-code shall be
validated, but only if the user filled the ISBN field:

class Book < ActiveRecord::Base
  # associations
  belongs_to :user, :counter_cache => true

  # validations
  validates_presence_of :title
  validates_format_of :isbn, :with => /[0-9\-a-zA-Z]{10}|[0-9\-a-zA-Z]
{13}/, :allow_nil => true
  validates_existence_of :user

  attr_protected :user_id
end

When I create a book on the console sans ISBN code, it works perfectly
fine:

b = Book.new

=> #<Book id: nil, title: nil, author: nil, isbn: nil, description:
nil, user_id: nil, created_at: nil, updated_at: nil>

b.user_id = 1

=> 1

b.title = "Book #1"

=> "Book #1"

b.save

=> true

When I enter a non isbn-10 or isbn-13 code, it properly raises the
error:

b.title = "Book #2"

=> "Book #2"

b.isbn = "abc"

=> "abc"

b.save

=> false

b.errors

=> #<ActiveRecord::Errors:0x2409564 @base=#<Book id: nil, title: "Book
#2", author: nil, isbn: "abc", description: nil, user_id: 1,
created_at: nil, updated_at: nil>, @errors={"isbn"=>["is invalid"]}>

So far so good. Now when I do the same in the browser & create an ISBN-
less book, it says:

1 error prohibited this book from being saved
There were problems with the following fields:
Isbn is invalid

And the params looking like this:
{"commit"=>"Create",
"authenticity_token"=>"5c27df353e84da4378277ff8201bcacdb394a0d4",
"action"=>"create", "controller"=>"books", "user_id"=>"1",
"book"=>{"isbn"=>"", "title"=>"abc", "author"=>"", "description"=>""}}

Why is this? Shouldn't :allow_nil work there, too?

Or did I miss something?
-J

not sure, but most likely "isbn"=>"" is an empty string and that for not
nil

at least erb:
"".nil?
=> false

Thorsten,

yeah - you're right.. didn't expect it that way entirely... but a
simple fix for this e.g. that one: http://opensoul.org/2007/2/7/validations-on-empty-not-nil-attributes