Drop key doesn't work well, drop incorrect key and add correct one.

db/install.xml

@@ -127,7 +127,7 @@
       </FIELDS>
       <KEYS>
         <KEY NAME="primary" TYPE="primary" FIELDS="id"/>
-        <KEY NAME="level_id" TYPE="unique" FIELDS="idnumber, warningpercent"/>
+        <KEY NAME="level_id" TYPE="unique" FIELDS="idnumber, warningpercent, warnafter"/>
       </KEYS>
     </TABLE>
     <TABLE NAME="attendance_warning_done" COMMENT="Warnings processed">

db/upgrade.php

@@ -335,28 +335,6 @@ function xmldb_attendance_upgrade($oldversion=0) {
     // Add new warning table.
     if ($oldversion < 2016121315) {
 
-        // Define table attendance_warning to be created.
-        $table = new xmldb_table('attendance_warning');
-
-        // Adding fields to table attendance_warning.
-        $table->add_field('id', XMLDB_TYPE_INTEGER, '10', null, XMLDB_NOTNULL, XMLDB_SEQUENCE, null);
-        $table->add_field('idnumber', XMLDB_TYPE_INTEGER, '10', null, XMLDB_NOTNULL, null, null);
-        $table->add_field('warningpercent', XMLDB_TYPE_INTEGER, '10', null, XMLDB_NOTNULL, null, null);
-        $table->add_field('warnafter', XMLDB_TYPE_INTEGER, '10', null, XMLDB_NOTNULL, null, null);
-        $table->add_field('emailuser', XMLDB_TYPE_INTEGER, '4', null, XMLDB_NOTNULL, null, null);
-        $table->add_field('emailsubject', XMLDB_TYPE_CHAR, '255', null, XMLDB_NOTNULL, null, null);
-        $table->add_field('emailcontent', XMLDB_TYPE_TEXT, null, null, XMLDB_NOTNULL, null, null);
-        $table->add_field('emailcontentformat', XMLDB_TYPE_INTEGER, '4', null, XMLDB_NOTNULL, null, null);
-        $table->add_field('thirdpartyemails', XMLDB_TYPE_TEXT, null, null, null, null, null);
-
-        // Adding keys to table attendance_warning.
-        $table->add_key('primary', XMLDB_KEY_PRIMARY, array('id'));
-
-        // Conditionally launch create table for attendance_warning.
-        if (!$dbman->table_exists($table)) {
-            $dbman->create_table($table);
-        }
-
         // Define table attendance_warning_done to be created.
         $table = new xmldb_table('attendance_warning_done');
 
@@ -381,17 +359,44 @@ function xmldb_attendance_upgrade($oldversion=0) {
         upgrade_mod_savepoint(true, 2016121315, 'attendance');
     }
 
-    if ($oldversion < 2016121317) {
+    if ($oldversion < 2016121318) {
         // Fix key.
         $table = new xmldb_table('attendance_warning');
-        if ($table->getkey('level_id')) {
-            $table->deleteKey('level_id');
-        }
-        $key = new xmldb_key('level_id', XMLDB_KEY_UNIQUE, array('idnumber, warningpercent, warnafter'));
-        $dbman->add_key($table, $key);
 
+        if (!$dbman->table_exists($table)) {
+            // Adding fields to table attendance_warning.
+            $table->add_field('id', XMLDB_TYPE_INTEGER, '10', null, XMLDB_NOTNULL, XMLDB_SEQUENCE, null);
+            $table->add_field('idnumber', XMLDB_TYPE_INTEGER, '10', null, XMLDB_NOTNULL, null, null);
+            $table->add_field('warningpercent', XMLDB_TYPE_INTEGER, '10', null, XMLDB_NOTNULL, null, null);
+            $table->add_field('warnafter', XMLDB_TYPE_INTEGER, '10', null, XMLDB_NOTNULL, null, null);
+            $table->add_field('emailuser', XMLDB_TYPE_INTEGER, '4', null, XMLDB_NOTNULL, null, null);
+            $table->add_field('emailsubject', XMLDB_TYPE_CHAR, '255', null, XMLDB_NOTNULL, null, null);
+            $table->add_field('emailcontent', XMLDB_TYPE_TEXT, null, null, XMLDB_NOTNULL, null, null);
+            $table->add_field('emailcontentformat', XMLDB_TYPE_INTEGER, '4', null, XMLDB_NOTNULL, null, null);
+            $table->add_field('thirdpartyemails', XMLDB_TYPE_TEXT, null, null, null, null, null);
+
+            // Adding keys to table attendance_warning.
+            $table->add_key('primary', XMLDB_KEY_PRIMARY, array('id'));
+            $table->add_key('level_id', XMLDB_KEY_UNIQUE, array('idnumber, warningpercent, warnafter'));
+
+            // Conditionally launch create table for attendance_warning.
+            $dbman->create_table($table);
+
+        } else {
+            // Key definition is probably incorrect so fix it - drop_key dml function doesn't seem to work.
+            $indexes = $DB->get_indexes('attendance_warning');
+            foreach ($indexes as $name => $index) {
+                if ($DB->get_dbfamily() === 'mysql') {
+                    $DB->execute("ALTER TABLE {attendance_warning} DROP INDEX ". $name);
+                } else {
+                    $DB->execute("DROP INDEX ". $name);
+                }
+            }
+            $index = new xmldb_key('level_id', XMLDB_KEY_UNIQUE, array('idnumber, warningpercent', 'warnafter'));
+            $dbman->add_key($table, $index);
+        }
         // Attendance savepoint reached.
-        upgrade_mod_savepoint(true, 2016121317, 'attendance');
+        upgrade_mod_savepoint(true, 2016121318, 'attendance');
     }
 
     return $result;

version.php

@@ -23,7 +23,7 @@
  */
 defined('MOODLE_INTERNAL') || die();
 
-$plugin->version  = 2016121317;
+$plugin->version  = 2016121318;
 $plugin->requires = 2016111800;
 $plugin->release = '3.2.14';
 $plugin->maturity  = MATURITY_STABLE;