diff --git a/lib/Simba/CA.pm b/lib/Simba/CA.pm
index d3809b4..57f6778 100644
--- a/lib/Simba/CA.pm
+++ b/lib/Simba/CA.pm
@@ -80,6 +80,7 @@ sub backup2disk {
 
     $self->{last_backup} = $dirs[-1];
     $self->{last_backup} = $1 if $self->{last_backup} =~ /(.*)/; # detaint
+    $self->{last_backup_id} = $self->get_last_session_id();
 
     my $timestamp = $self->{timestamp} || strftime('%Y-%m-%dT%H.%M.%S', localtime);
     $self->{this_backup} = $self->{basedir} . "/$timestamp/" . $target->{host} . '/' .  $target->{dir};
@@ -374,6 +375,14 @@ sub db_record_version {
                                                     $target->{id}, $f->{name});
 
     }
+    if ($f->{t} eq 'f' && !defined($f->{checksum})) {
+        # this must be a link to the previous version
+        my $db_pv = $self->{dbh}->selectall_arrayref("select * from versions where file=? and session=?",
+                                                     { Slice => {} },
+                                                     $db_f->[0]{id},
+                                                     $self->{last_backup_id});
+        $f->{checksum} = $db_pv->[0]{checksum};
+    }
     $self->{dbh}->do("insert into versions(file,
                                            file_id, file_type, file_size, file_mtime,
                                            file_owner, file_group, file_acl,
@@ -410,5 +419,14 @@ sub close_session {
     $self->{dbh}->do("update sessions set end_date=? where id=?", {}, time(), $self->{session_id});
 }
 
+sub get_last_session_id {
+    my ($self) = @_;
+    my $sessions = $self->{dbh}->selectall_arrayref("select * from sessions where prefix=?",
+                                                    { Slice => {} },
+                                                    $self->{last_backup});
+    die "$self->{last_backup} not a unique prefix" unless @$sessions == 1;
+    return $sessions->[0]{id};
+}
+
 # vim: tw=0 expandtab
 1;