For those times when you want to get a date object out of the multi-part parameters without having to do a mass assignment to a model object. I extracted this code directly from AR so I think it should be pretty solid but I have not tested it so your mileage may vary.

Take the following and place it into your application helper or whatever helper you'd like. To use it you simply pass in an instance of the object on which the multi-part parameter exists. This is so that the type to instantiate can be reflected from the column information. Then you specify the attribute name of the parameter you're looking to create from the next parameter which is the hash containing the multi-part parameter value.

example:
for a form containing dude[birth_date] you'd call the method like this:

extract_date(@the_dude, 'birth_date', params[:dude])

   1  
   2      def extract_date(parent_object, param_name, atts)
   3        atts.stringify_keys!
   4  
   5        multi_parameter_attributes = []
   6        atts.each do |k, v|
   7          multi_parameter_attributes << [ k, v ] if k.include?(param_name+"(")
   8        end
   9        
  10        attributes = { }
  11  
  12        for pair in multi_parameter_attributes
  13          multiparameter_name, value = pair
  14          attribute_name = multiparameter_name.split("(").first
  15          attributes[attribute_name] = [] unless attributes.include?(attribute_name)
  16  
  17          unless value.empty?
  18            attributes[attribute_name] <<
  19              [ find_parameter_position(multiparameter_name), type_cast_attribute_value(multiparameter_name, value) ]
  20          end
  21        end
  22  
  23        attributes.each { |name, values| attributes[name] = values.sort_by{ |v| v.first }.collect { |v| v.last } }
  24        errors = []
  25        result = nil
  26        attributes.each do |name, values|  
  27          klass = (parent_object.class.reflect_on_aggregation(name.to_sym) || parent_object.column_for_attribute(name)).klass
  28          result = Time == klass ? klass.local(*values) : klass.new(*values)
  29        end
  30  
  31        return result
  32      end
  33      
  34      def find_parameter_position(multiparameter_name)
  35        multiparameter_name.scan(/\(([0-9]*).*\)/).first.first
  36      end
  37  
  38      def type_cast_attribute_value(multiparameter_name, value)
  39        multiparameter_name =~ /\([0-9]*([a-z])\)/ ? value.send("to_" + $1) : value
  40      end