# Will ensure the set of provided attributes is unique - ie: no other record
# should have the same combination of values for the same attributes.
# 
# Example:
#   validates_uniqueness_of_set :first_name, :last_name
# 
# Written by Pat Allan (http://freelancing-gods.com), and based heavily on
# Rails' validates_uniqueness_of code.
# 
def validates_uniqueness_of_set(*attr_names)
  configuration = {
    :message => ActiveRecord::Errors.default_error_messages[:taken],
    :case_sensitive => true
  }
  configuration.update(attr_names.extract_options!)

  validate do |record|
    condition_sql    = []
    condition_params = []

    attr_names.each do |attr_name|
      value = record.send(attr_name)

      if value.nil? || (configuration[:case_sensitive] || !columns_hash[attr_name.to_s].text?)
        condition_sql << "#{record.class.table_name}.#{attr_name} #{attribute_condition(value)}"
        condition_params << value
      else
        condition_sql << "LOWER(#{record.class.table_name}.#{attr_name}) #{attribute_condition(value)}"
        condition_params << value.downcase
      end
    end

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

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

    # 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 finder_class.find(:first, :conditions => [condition_sql.join(" AND "), *condition_params])
      record.errors.add(attr_names.first, configuration[:message])
    end
  end
end