diff --git a/mod/workshop/db/upgrade.php b/mod/workshop/db/upgrade.php
index 13f02d84200..df614c921d9 100644
--- a/mod/workshop/db/upgrade.php
+++ b/mod/workshop/db/upgrade.php
@@ -264,5 +264,49 @@ function xmldb_workshop_upgrade($oldversion) {
         upgrade_mod_savepoint(true, 2010111200, 'workshop');
     }
 
+    /**
+     * Check the course_module integrity - see MDL-26312 for details
+     *
+     * Because of a bug in Workshop upgrade code, multiple workshop course_modules can
+     * potentially point to a single workshop instance. The chance is pretty low as in most cases,
+     * the upgrade failed. But under certain circumstances, workshop could be upgraded with
+     * this data integrity issue. We want to detect it now and let the admin know.
+     */
+    if ($oldversion < 2011021100) {
+        $sql = "SELECT cm.id, cm.course, cm.instance
+                  FROM {course_modules} cm
+                 WHERE cm.module IN (SELECT id
+                                       FROM {modules}
+                                      WHERE name = ?)";
+        $rs = $DB->get_recordset_sql($sql, array('workshop'));
+        $map = array(); // returned stdClasses by instance id
+        foreach ($rs as $cm) {
+            $map[$cm->instance][$cm->id] = $cm;
+        }
+        $rs->close();
+
+        $problems = array();
+        foreach ($map as $instanceid => $cms) {
+            if (count($cms) > 1) {
+                $problems[] = 'workshop instance ' . $instanceid . ' referenced by course_modules ' . implode(', ', array_keys($cms));
+            }
+        }
+        if ($problems) {
+            echo $OUTPUT->notification('¡Ay, caramba! Data integrity corruption has been detected in your workshop ' . PHP_EOL .
+                'module database tables. This might be caused by a bug in workshop upgrade code. ' . PHP_EOL .
+                'Please report this issue immediately in workshop module support forum at ' . PHP_EOL .
+                'http://moodle.org so that we can help to fix this problem. Please copy and keep ' . PHP_EOL .
+                'following information for future reference:');
+            foreach ($problems as $problem) {
+                echo $OUTPUT->notification($problem);
+                upgrade_log(UPGRADE_LOG_NOTICE, 'mod_workshop', 'course_modules integrity problem', $problem);
+            }
+        }
+
+        unset($problems);
+        unset($map);
+        upgrade_mod_savepoint(true, 2011021100, 'workshop');
+    }
+
     return true;
 }
diff --git a/mod/workshop/db/upgradelib.php b/mod/workshop/db/upgradelib.php
index 8bc77f44048..4b79846f45a 100644
--- a/mod/workshop/db/upgradelib.php
+++ b/mod/workshop/db/upgradelib.php
@@ -154,13 +154,13 @@ function workshop_upgrade_module_instances() {
 
     upgrade_set_timeout();
     $moduleid = $DB->get_field('modules', 'id', array('name' => 'workshop'), MUST_EXIST);
-    $rs = $DB->get_recordset_select('workshop_old', 'newid IS NULL');
+    $rs = $DB->get_recordset_select('workshop_old', 'newid IS NULL', null, 'id');
     foreach ($rs as $old) {
         $new = workshop_upgrade_transform_instance($old);
-        $newid = $DB->insert_record('workshop', $new, true, true);
-        $DB->set_field('course_modules', 'instance', $newid, array('module' => $moduleid, 'instance' => $old->id));
+        $new->id = $old->id;
+        $DB->import_record('workshop', $new);
         $DB->set_field('workshop_old', 'newplugin', 'workshop', array('id' => $old->id));
-        $DB->set_field('workshop_old', 'newid', $newid, array('id' => $old->id));
+        $DB->set_field('workshop_old', 'newid', $new->id, array('id' => $old->id));
     }
     $rs->close();
 }
@@ -214,24 +214,6 @@ function workshop_upgrade_transform_instance(stdClass $old) {
     return $new;
 }
 
-/**
- * Returns the list of new workshop instances ids
- *
- * @return array (int)oldid => (int)newid
- */
-function workshop_upgrade_workshop_id_mappings() {
-    global $DB;
-
-    $oldrecords = $DB->get_records('workshop_old', null, 'id', 'id,newid');
-    $newids = array();
-    foreach ($oldrecords as $oldid => $oldrecord) {
-        if ($oldrecord->id and $oldrecord->newid) {
-            $newids[$oldid] = $oldrecord->newid;
-        }
-    }
-    return $newids;
-}
-
 /**
  * Copies records from workshop_submissions_old into workshop_submissions. Can be called after all workshop module instances
  * were correctly migrated and new ids are filled in workshop_old
@@ -242,19 +224,18 @@ function workshop_upgrade_submissions() {
     global $CFG, $DB;
 
     upgrade_set_timeout();
-    $newworkshopids = workshop_upgrade_workshop_id_mappings();
 
-    // list of teachers in every workshop: array of (int)oldworkshopid => array of (int)userid => notused
+    // list of teachers in every workshop: array of (int)workshopid => array of (int)userid => notused
     $workshopteachers = array();
 
     $rs = $DB->get_recordset_select('workshop_submissions_old', 'newid IS NULL');
     foreach ($rs as $old) {
         if (!isset($workshopteachers[$old->workshopid])) {
-            $cm = get_coursemodule_from_instance('workshop', $newworkshopids[$old->workshopid], 0, false, MUST_EXIST);
+            $cm = get_coursemodule_from_instance('workshop', $old->workshopid, 0, false, MUST_EXIST);
             $context = get_context_instance(CONTEXT_MODULE, $cm->id);
             $workshopteachers[$old->workshopid] = get_users_by_capability($context, 'mod/workshop:manage', 'u.id');
         }
-        $new = workshop_upgrade_transform_submission($old, $newworkshopids[$old->workshopid], $workshopteachers[$old->workshopid]);
+        $new = workshop_upgrade_transform_submission($old, $old->workshopid, $workshopteachers[$old->workshopid]);
         $newid = $DB->insert_record('workshop_submissions', $new, true, true);
         $DB->set_field('workshop_submissions_old', 'newplugin', 'submissions', array('id' => $old->id));
         $DB->set_field('workshop_submissions_old', 'newid', $newid, array('id' => $old->id));
@@ -343,17 +324,17 @@ function workshop_upgrade_assessments() {
     global $CFG, $DB, $OUTPUT;
 
     upgrade_set_timeout();
-    $newworkshopids     = workshop_upgrade_workshop_id_mappings();
+
     $newsubmissionids   = workshop_upgrade_submission_id_mappings();
     $teacherweights     = workshop_upgrade_legacy_teacher_weights();
 
-    // list of teachers in every workshop: array of (int)oldworkshopid => array of (int)userid => notused
+    // list of teachers in every workshop: array of (int)workshopid => array of (int)userid => notused
     $workshopteachers   = array();
 
     $rs = $DB->get_recordset_select('workshop_assessments_old', 'newid IS NULL');
     foreach ($rs as $old) {
         if (!isset($workshopteachers[$old->workshopid])) {
-            $cm = get_coursemodule_from_instance('workshop', $newworkshopids[$old->workshopid], 0, false, MUST_EXIST);
+            $cm = get_coursemodule_from_instance('workshop', $old->workshopid, 0, false, MUST_EXIST);
             $context = get_context_instance(CONTEXT_MODULE, $cm->id);
             $workshopteachers[$old->workshopid] = get_users_by_capability($context, 'mod/workshop:manage', 'u.id');
         }
diff --git a/mod/workshop/form/accumulative/db/upgradelib.php b/mod/workshop/form/accumulative/db/upgradelib.php
index 3376c864067..1e7237ec86d 100644
--- a/mod/workshop/form/accumulative/db/upgradelib.php
+++ b/mod/workshop/form/accumulative/db/upgradelib.php
@@ -49,11 +49,10 @@ function workshopform_accumulative_upgrade_legacy() {
                  WHERE workshopid $workshopids
                        AND newid IS NULL";
         $rs = $DB->get_recordset_sql($sql, $params);
-        $newworkshopids = workshop_upgrade_workshop_id_mappings();
         // prepare system (global) scales to replace the legacy in-built ones
         $newscaleids = workshopform_accumulative_upgrade_scales();
         foreach ($rs as $old) {
-            $new = workshopform_accumulative_upgrade_element($old, $newscaleids, $newworkshopids[$old->workshopid]);
+            $new = workshopform_accumulative_upgrade_element($old, $newscaleids, $old->workshopid);
             $newid = $DB->insert_record('workshopform_accumulative', $new);
             $DB->set_field('workshop_elements_old', 'newplugin', 'accumulative', array('id' => $old->id));
             $DB->set_field('workshop_elements_old', 'newid', $newid, array('id' => $old->id));
diff --git a/mod/workshop/form/comments/db/upgradelib.php b/mod/workshop/form/comments/db/upgradelib.php
index f1e537bb6e1..ee45f2ad142 100644
--- a/mod/workshop/form/comments/db/upgradelib.php
+++ b/mod/workshop/form/comments/db/upgradelib.php
@@ -49,9 +49,8 @@ function workshopform_comments_upgrade_legacy() {
                  WHERE workshopid $workshopids
                        AND newid IS NULL";
         $rs = $DB->get_recordset_sql($sql, $params);
-        $newworkshopids = workshop_upgrade_workshop_id_mappings();
         foreach ($rs as $old) {
-            $new = workshopform_comments_upgrade_element($old, $newworkshopids[$old->workshopid]);
+            $new = workshopform_comments_upgrade_element($old, $old->workshopid);
             $newid = $DB->insert_record('workshopform_comments', $new);
             $DB->set_field('workshop_elements_old', 'newplugin', 'comments', array('id' => $old->id));
             $DB->set_field('workshop_elements_old', 'newid', $newid, array('id' => $old->id));
diff --git a/mod/workshop/form/numerrors/db/upgradelib.php b/mod/workshop/form/numerrors/db/upgradelib.php
index f7b07f86ddb..e27d61e303d 100644
--- a/mod/workshop/form/numerrors/db/upgradelib.php
+++ b/mod/workshop/form/numerrors/db/upgradelib.php
@@ -53,11 +53,10 @@ function workshopform_numerrors_upgrade_legacy() {
                  WHERE workshopid $workshopids
                        AND newid IS NULL";
         $rs = $DB->get_recordset_sql($sql, $params);
-        $newworkshopids = workshop_upgrade_workshop_id_mappings();
         foreach ($rs as $old) {
             // process the information about mapping
             $newmapping = new stdclass();
-            $newmapping->workshopid = $newworkshopids[$old->workshopid];
+            $newmapping->workshopid = $old->workshopid;
             $newmapping->nonegative = $old->elementno;
             $newmapping->grade = $old->maxscore;
             if ($old->maxscore > 0) {
@@ -70,7 +69,7 @@ function workshopform_numerrors_upgrade_legacy() {
             $DB->insert_record('workshopform_numerrors_map', $newmapping);
             // process the information about the element itself
             if (trim($old->description) and $old->description <> '@@ GRADE_MAPPING_ELEMENT @@') {
-                $new = workshopform_numerrors_upgrade_element($old, $newworkshopids[$old->workshopid]);
+                $new = workshopform_numerrors_upgrade_element($old, $old->workshopid);
                 $newid = $DB->insert_record('workshopform_numerrors', $new);
             } else {
                 $newid = 0;
diff --git a/mod/workshop/form/rubric/db/upgradelib.php b/mod/workshop/form/rubric/db/upgradelib.php
index 40df767619e..50c4fbde447 100644
--- a/mod/workshop/form/rubric/db/upgradelib.php
+++ b/mod/workshop/form/rubric/db/upgradelib.php
@@ -57,25 +57,24 @@ function workshopform_rubric_upgrade_legacy_criterion() {
                  WHERE workshopid $workshopids
                        AND newid IS NULL";
         $rs = $DB->get_recordset_sql($sql, $params);
-        $newworkshopids = workshop_upgrade_workshop_id_mappings();
-        $newdimensionids = array(); // (int)oldworkshopid => (int)dimensionid
+        $newdimensionids = array(); // (int)workshopid => (int)dimensionid
         foreach ($rs as $old) {
             // create rubric criterion and the configuration if necessary
             if (!isset($newdimensionids[$old->workshopid])) {
-                if (!$DB->record_exists('workshopform_rubric', array('workshopid' => $newworkshopids[$old->workshopid], 'sort' => 1))) {
+                if (!$DB->record_exists('workshopform_rubric', array('workshopid' => $old->workshopid, 'sort' => 1))) {
                     $newdimension = new stdclass();
-                    $newdimension->workshopid = $newworkshopids[$old->workshopid];
+                    $newdimension->workshopid = $old->workshopid;
                     $newdimension->sort = 1;
                     $newdimension->description = trim(get_string('dimensionnumber', 'workshopform_rubric', ''));
                     $newdimension->descriptionformat = FORMAT_HTML;
                     $newdimensionids[$old->workshopid] = $DB->insert_record('workshopform_rubric', $newdimension);
                 } else {
                     $newdimensionids[$old->workshopid] = $DB->get_field('workshopform_rubric', 'id',
-                                                                array('workshopid' => $newworkshopids[$old->workshopid], 'sort' => 1));
+                                                                array('workshopid' => $old->workshopid, 'sort' => 1));
                 }
-                if (!$DB->record_exists('workshopform_rubric_config', array('workshopid' => $newworkshopids[$old->workshopid]))) {
+                if (!$DB->record_exists('workshopform_rubric_config', array('workshopid' => $old->workshopid))) {
                     $newconfig = new stdclass();
-                    $newconfig->workshopid = $newworkshopids[$old->workshopid];
+                    $newconfig->workshopid = $old->workshopid;
                     $newconfig->layout = 'list';
                     $DB->insert_record('workshopform_rubric_config', $newconfig);
                 }
@@ -148,27 +147,26 @@ function workshopform_rubric_upgrade_legacy_rubric() {
                        AND r.newid IS NULL
               ORDER BY e.workshopid, e.elementno, r.rubricno";
         $rs = $DB->get_recordset_sql($sql, $params);
-        $newworkshopids     = workshop_upgrade_workshop_id_mappings();  // array of (int)oldworkshopid => (int)newworkshopid
-        $newdimensionids    = array();  // (int)oldworkshopid => (int)elementno => (int)dimensionid
+        $newdimensionids    = array();  // (int)workshopid => (int)elementno => (int)dimensionid
         $newlevelids        = array();  // (int)oldrubricid => (int)newlevelid
         $prevelement        = null;
         foreach ($rs as $old) {
             // create rubric criterion and the configuration if necessary
             if (!isset($newdimensionids[$old->workshopid]) or !isset($newdimensionids[$old->workshopid][$old->esort])) {
-                if (!$DB->record_exists('workshopform_rubric', array('workshopid' => $newworkshopids[$old->workshopid], 'sort' => $old->esort))) {
+                if (!$DB->record_exists('workshopform_rubric', array('workshopid' => $old->workshopid, 'sort' => $old->esort))) {
                     $newdimension = new stdclass();
-                    $newdimension->workshopid = $newworkshopids[$old->workshopid];
+                    $newdimension->workshopid = $old->workshopid;
                     $newdimension->sort = $old->esort;
                     $newdimension->description = $old->edesc;
                     $newdimension->descriptionformat = FORMAT_HTML;
                     $newdimensionids[$old->workshopid][$old->esort] = $DB->insert_record('workshopform_rubric', $newdimension);
                 } else {
                     $newdimensionids[$old->workshopid][$old->esort] = $DB->get_field('workshopform_rubric', 'id',
-                                                                    array('workshopid' => $newworkshopids[$old->workshopid], 'sort' => $old->esort));
+                                                                    array('workshopid' => $old->workshopid, 'sort' => $old->esort));
                 }
-                if (!$DB->record_exists('workshopform_rubric_config', array('workshopid' => $newworkshopids[$old->workshopid]))) {
+                if (!$DB->record_exists('workshopform_rubric_config', array('workshopid' => $old->workshopid))) {
                     $newconfig = new stdclass();
-                    $newconfig->workshopid = $newworkshopids[$old->workshopid];
+                    $newconfig->workshopid = $old->workshopid;
                     $newconfig->layout = 'grid';
                     $DB->insert_record('workshopform_rubric_config', $newconfig);
                 }
diff --git a/mod/workshop/version.php b/mod/workshop/version.php
index 9cc8d8871ef..11348c1c9f6 100644
--- a/mod/workshop/version.php
+++ b/mod/workshop/version.php
@@ -29,6 +29,6 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$module->version  = 2010111200;
-$module->requires = 2010111002;  // Requires this Moodle version
+$module->version  = 2011021100;
+$module->requires = 2011020900;  // Requires this Moodle version
 //$module->cron     = 60;