    # File lib/active_record/validations.rb, line 598
          def validates_uniqueness_of(*attr_names)
            configuration = { :message => ActiveRecord::Errors.default_error_messages[:taken], :case_sensitive => true }
            configuration.update(attr_names.extract_options!)

            validates_each(attr_names,configuration) do |record, attr_name, value|
              # The check for an existing value should be run from a class that
              # isn't abstract. This means working down from the current class
              # (self), to the first non-abstract class. Since classes don't know
              # their subclasses, we have to build the hierarchy between self and
              # the record's class.
              class_hierarchy = [record.class]
              while class_hierarchy.first != self
                class_hierarchy.insert(0, class_hierarchy.first.superclass)
              end

              # Now we can work our way down the tree to the first non-abstract
              # class (which has a database table to query from).
              finder_class = class_hierarchy.detect { |klass| !klass.abstract_class? }

              if value.nil? || (configuration[:case_sensitive] || !finder_class.columns_hash[attr_name.to_s].text?)
                condition_sql = "#{record.class.quoted_table_name}.#{attr_name} #{attribute_condition(value)}"
                condition_params = [value]
              else
                # sqlite has case sensitive SELECT query, while MySQL/Postgresql don't.
                # Hence, this is needed only for sqlite.
                condition_sql = "LOWER(#{record.class.quoted_table_name}.#{attr_name}) #{attribute_condition(value)}"
                condition_params = [value.downcase]
              end

              if scope = configuration[:scope]
                Array(scope).map do |scope_item|
                  scope_value = record.send(scope_item)
                  condition_sql << " AND #{record.class.quoted_table_name}.#{scope_item} #{attribute_condition(scope_value)}"
                  condition_params << scope_value
                end
              end

              unless record.new_record?
                condition_sql << " AND #{record.class.quoted_table_name}.#{record.class.primary_key} <> ?"
                condition_params << record.send(:id)
              end

              results = finder_class.with_exclusive_scope do
                connection.select_all(
                  construct_finder_sql(
                    :select     => "#{connection.quote_column_name(attr_name)}",
                    :from       => "#{finder_class.quoted_table_name}",
                    :conditions => [condition_sql, *condition_params]
                  )
                )
              end

              unless results.length.zero?
                found = true

                # As MySQL/Postgres don't have case sensitive SELECT queries, we try to find duplicate
                # column in ruby when case sensitive option
                if configuration[:case_sensitive] && finder_class.columns_hash[attr_name.to_s].text?
                  found = results.any? { |a| a[attr_name.to_s] == value }
                end

                record.errors.add(attr_name, configuration[:message]) if found
              end
            end
          end