Accessing params in validations - ruby-on-rails-3

Question

I would like to show some extra info in error messages resulting from a failed validation. For example suppose I have a class Book with the following validation
validates :name, presence: true, uniqueness: true
When someone tries to insert a book by the same name the following error message is returned
{"name":["has already been taken"]}
Instead I wanna show
{"name":["Book 'Great Expectaions' has already been taken at id:7"]}
Right now to make this happen I have to remove the uniqueness validation that I mentioned above and do the following
validate do |book|
existing_book = Book.find_by_name(book.name)
if existing_book
book.errors.add(:name, "#{existing_book.name} already exists at id: #{existing_book.id}")
end
end
Is there a way to get custom error messages like above without writing a custom uniqueness validation? I was thinking something along the lines of
validates :name, presence: true, uniqueness: {message: "#{self.name} already exists at id: #{Book.find_by_name(self.name).id}"
But this does not seem to work as self.name returns 'Book'. Is there a way to access the passed parameters in this context?

Answer 1

You'll have to do this as a custom validation. I would do it like so:
validate :name_is_unique
private
def name_is_unique
errors.add(:name, "#{other_book.name} already exists at id: #{other_book.id}") if other_book = Book.find_by_name(name)
end
The issue isn't really that you can't include the current model attributes in your validation, its that there's no 'one-liner' way to include another model. The good news is, that's what the validate method is for.
If it bothers you to have this in your model, just write a custom validator so it can be re-used application-wide.