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# app/models/post.rb
class Post < ActiveRecord::Base
validate :ensure_slug_uniqueness
protected
# validate
def ensure_slug_uniqueness
# we also want to ensure the slug is not blank
if self.slug.blank?
errors.add(:slug, "can't be blank")
end
# if this is a new post, the id is nil
# otherwise, the slug should point to this post's id
unless Slug[self.slug].nil? || Slug[self.slug] == self.id.to_s
errors.add(:slug, "is already taken")
end
end
end
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