Index: openacs-4/packages/xowiki/tcl/xowiki-utility-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/xowiki/tcl/xowiki-utility-procs.tcl,v
diff -u -r1.37.2.1 -r1.37.2.2
--- openacs-4/packages/xowiki/tcl/xowiki-utility-procs.tcl	14 Aug 2015 17:25:59 -0000	1.37.2.1
+++ openacs-4/packages/xowiki/tcl/xowiki-utility-procs.tcl	20 Aug 2015 17:09:51 -0000	1.37.2.2
@@ -465,7 +465,7 @@
       set tn [$cl table_name]
       set cn ${tn}_fk
       set sc [$cl info superclass]
-      ::xo::dc dml drop_constraint "ALTER TABLE $tn DROP constraint $cn"
+      ::xo::dc dml drop_constraint "ALTER TABLE $tn DROP constraint IF EXISTS $cn"
       ::xo::dc dml add_constraint  "ALTER TABLE $tn ADD constraint $cn FOREIGN KEY([$cl id_column]) \
         REFERENCES [$sc table_name]([$sc id_column]) ON DELETE CASCADE"
     }