```ruby
require_dependency 'issue'

module SerialNumberField
  module IssuePatch
    extend ActiveSupport::Concern

    def assign_serial_number!
      serial_number_fields.each do |cf|
        next if assigned_serial_number?(cf)

        target_custom_value = serial_number_custom_values(cf).first
        target_custom_value.update_attributes!(
          :value => cf.format.generate_value(cf, self))
      end
    end

    def assigned_serial_number?(cf)
      serial_number_custom_values(cf).where(
        'custom_values.value is not null').present?
    end

    def serial_number_custom_values(cf)
      CustomValue.where(:custom_field_id => cf.id,
        :customized_type => 'Issue',
        :customized_id => self.id)
    end

    def serial_number_fields
      editable_custom_fields.select do |value|
        value.field_format == SerialNumberField::Format::NAME
      end
    end

  end
end

unless Issue.included_modules.include?(SerialNumberField::IssuePatch)
  Issue.send(:include, SerialNumberField::IssuePatch)
end
```

Fix not have a CustomValue of serial number Error
user
```ruby
require_dependency 'issue'

module SerialNumberField
  module IssuePatch
    extend ActiveSupport::Concern

    def assign_serial_number!
      serial_number_fields.each do |cf|
        next if assigned_serial_number?(cf)

        target_custom_value = serial_number_custom_values(cf).first
        new_serial_number = cf.format.generate_value(cf, self)

        # TODO find_or_create_by
        if target_custom_value.present?
          target_custom_value.update_attributes!(
            :value => new_serial_number)
        else
          CustomValue.create!(:custom_field_id => cf.id,
            :customized_type => 'Issue',
            :customized_id => self.id,
            :value => new_serial_number)
        end
      end
    end

    def assigned_serial_number?(cf)
      serial_number_custom_values(cf).where(
        'custom_values.value is not null').present?
    end

    def serial_number_custom_values(cf)
      CustomValue.where(:custom_field_id => cf.id,
        :customized_type => 'Issue',
        :customized_id => self.id)
    end

    def serial_number_fields
      editable_custom_fields.select do |value|
        value.field_format == SerialNumberField::Format::NAME
      end
    end

  end
end

unless Issue.included_modules.include?(SerialNumberField::IssuePatch)
  Issue.send(:include, SerialNumberField::IssuePatch)
end
```
assistant