Redmine

The sorting by multiple value field is disabled by:
app\models\custom_field.rb, line 193:

  # Returns a ORDER BY clause that can used to sort customized
  # objects by their value of the custom field.
  # Returns nil if the custom field can not be used for sorting.
  def order_statement
    return nil if multiple?
    format.order_statement(self)
  end

But actually we can sorting by the multiple value allowed fields by:
1. order/sort the values of the multiple value allowed fields firstly, and generate the string;
2. order/sort by the field.