added 'uploads:backfill_shas' rake task

lib/tasks/uploads.rake

@@ -0,0 +1,20 @@
+require "digest/sha1"
+
+task "uploads:backfill_shas" => :environment do
+  RailsMultisite::ConnectionManagement.each_connection do |db|
+    puts "Backfilling #{db}"
+    Upload.select([:id, :sha, :url]).find_each do |u|
+      if u.sha.nil?
+        putc "."
+        path = "#{Rails.root}/public/#{u.url}"
+        sha = Digest::SHA1.file(path).hexdigest
+        begin
+          Upload.update_all ["sha = ?", sha], ["id = ?", u.id]
+        rescue ActiveRecord::RecordNotUnique
+          # not a big deal if we've got a few duplicates
+        end
+      end
+    end
+  end
+  puts "done"
+end