diff --git a/mod/assign/feedback/editpdf/db/install.xml b/mod/assign/feedback/editpdf/db/install.xml
index e5c8007bbb5..3180246a8f2 100644
--- a/mod/assign/feedback/editpdf/db/install.xml
+++ b/mod/assign/feedback/editpdf/db/install.xml
@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" ?>
-<XMLDB PATH="mod/assign/feedback/editpdf/db" VERSION="20160216" COMMENT="XMLDB file for Moodle mod/assign/feedback/editpdf"
+<XMLDB PATH="mod/assign/feedback/editpdf/db" VERSION="20180925" COMMENT="XMLDB file for Moodle mod/assign/feedback/editpdf"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="../../../../../lib/xmldb/xmldb.xsd"
 >
@@ -68,6 +68,7 @@
       </FIELDS>
       <KEYS>
         <KEY NAME="primary" TYPE="primary" FIELDS="id"/>
+        <KEY NAME="submissionid-submissionattempt" TYPE="unique" FIELDS="submissionid, submissionattempt"/>
       </KEYS>
     </TABLE>
   </TABLES>