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.