Update Table column
===

I use datamapper for the database. I have a table.

    class ZedTable
      include DataMapper::Resource
      property :id,         Serial
      property :label,       String 
      property :now,    Boolean, :default => false  
      
      before :save do 
        ZedTable.all.update(:now => false)
        self.now = true
      end
    end

That is, I want only one value was `true`. But when I save the data I get an error.

    Failure/Error: Unable to find matching line from backtrace
     SystemStackError:
       stack level too deep

Why? And how do I solve this problem?
Thanks.