About query database and virtual attributes

Hi,

My model has a lot of virtual attributes. So when I use
find(:all, :condition=>blablabla),
can I use virtual attribute directly?

eg. in my model:
def mydatetime
   self.date+self.time
end
can i just use "mydatetime=xxx" to match the records in db?

of course, my model is far more complicated than this.

Any idea?