# File lib/active_record/validations.rb, line 704
      def validates_uniqueness_of(*attr_names)
        configuration = { :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? }

          is_text_column = finder_class.columns_hash[attr_name.to_s].text?

          if value.nil?
            comparison_operator = "IS ?"
          elsif is_text_column
            comparison_operator = "#{connection.case_sensitive_equality_operator} ?"
            value = value.to_s
          else
            comparison_operator = "= ?"
          end

          sql_attribute = "#{record.class.quoted_table_name}.#{connection.quote_column_name(attr_name)}"

          if value.nil? || (configuration[:case_sensitive] || !is_text_column)
            condition_sql = "#{sql_attribute} #{comparison_operator}"
            condition_params = [value]
          else
            condition_sql = "LOWER(#{sql_attribute}) #{comparison_operator}"
            condition_params = [value.mb_chars.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

          finder_class.with_exclusive_scope do
            if finder_class.exists?([condition_sql, *condition_params])
              record.errors.add(attr_name, :taken, :default => configuration[:message], :value => value)
            end
          end
        end
      end