[Rails] help finding database items that lost parent item

I have a table for private messages similar to:

PMS:
title
body
sent_to (foreign key to users table)
sent_from (foreign key to users table)

When I originally set it up I didn't have it automatically a users pm's
when that user was deleted. So...I have basically been manually
updating the database when a users has problems with their private
messages page.

I'm wondering if someone knows a good way I could scan the the pm's
table and delete messages that no longer have a user for sent_to or
sent_from.

I suppose this would probably best be done with a rake task but I'm
unsure of the condition syntax to check for the existence of the user.
Here is what I started to write:

task(:deletePMs => :environment) do
@pms = Pms.find(:all, :conditions => '?????')

@pms.each do |pm|
pm.destroy
puts "PM => " + pm.id + " destroyed!"
end
end
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