Automatically populating models with the defaults from the table definition.

I am using SQL server with ODBC adapter. If I have a field defined as
"hair_color string(255) default("UNK"), not null" it causes
problems with saving the record if I don't specify the hair color.

In this case if I don't specify a hair color AR puts a NULL into the
insert clause and SQL server pukes. If AR simply had left that field
out of the insert everything would have been fine.

Is there a way to tell AR to either leave the NULL fields out of the
insert (or update) or to load the defaults into the model when it's