class ActiveFedora::Base

m.update_attributes({:fubar=>'baz'}, :datastreams=>["my_ds", "my_other_ds"])
or
m.update_attributes({:fubar=>'baz'}, :datastreams=>"my_ds")
use the :datastreams argument like so:
If you want to specify which datastream(s) to update,

are _both_ overwritten.
the object's datastreams. This means DS1.fubar_values and DS2.fubar_values
This will attempt to set the values for any fields named fubar in any of

m.update_attributes(:fubar=>'baz')

Example Usage:

An ActiveRecord-ism to udpate metadata values.

def update_attributes(params={}, opts={})
  params.each do |k,v|
    if v == :delete || v == "" || v == nil
      v = []
    end
    if opts[:datastreams]
      ds_array = []
      opts[:datastreams].each do |dsname|
        ds_array << datastreams[dsname]
      end
    else
      ds_array = datastreams.values
    end
    ds_array.each do |d|
      if d.fields[k.to_sym]
        d.send("#{k}_values=", v)
      end
    end
  end
end