validates_uniqueness_of (ActiveRecord::Validations::ClassMethods)

    # File lib/active_record/validations.rb, line 773
          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? }

              column = finder_class.columns_hash[attr_name.to_s]

              if value.nil?
                comparison_operator = "IS ?"
              elsif column.text?
                comparison_operator = "#{connection.case_sensitive_equality_operator} ?"
                value = column.limit ? value.to_s.mb_chars[0, column.limit] : 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] || !column.text?)
                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 " << attribute_condition("#{record.class.quoted_table_name}.#{scope_item}", 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