class ActiveRecord::Validations::UniquenessValidator

def build_relation(klass, attribute, value)
  relation = klass.unscoped
  # TODO: Add case-sensitive / case-insensitive operators to Arel
  # to no longer need to checkout a connection here.
  comparison = klass.with_connection do |connection|
    relation.bind_attribute(attribute, value) do |attr, bind|
      return relation.none! if bind.unboundable?
      if !options.key?(:case_sensitive) || bind.nil?
        connection.default_uniqueness_comparison(attr, bind)
      elsif options[:case_sensitive]
        connection.case_sensitive_comparison(attr, bind)
      else
        # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
        connection.case_insensitive_comparison(attr, bind)
      end
    end
  end
  relation.where!(comparison)
end