MDL-65033 mod_forum: Modified sort sql for cross db support

mod/forum/classes/local/vaults/discussion_list.php

@@ -217,7 +217,19 @@ public function get_sort_order(?int $sortmethod, $includefavourites = true) : st
             }
         }
 
-        $favouritesort = ($includefavourites ? ", {$this->get_favourite_alias()}.id DESC" : "");
+        $favouritesort = '';
+        if ($includefavourites) {
+            $favalias = $this->get_favourite_alias();
+            // Since we're joining on the favourite table any discussion that isn't favourited will have
+            // null in the favourite columns. Nulls behave differently in the sorting for different databases.
+            // We can ensure consistency between databases by explicitly deprioritising any null favourite field
+            // using a case statement.
+            $favouritesort = ", CASE WHEN {$favalias}.id IS NULL THEN 0 ELSE 1 END DESC";
+            // After the null favourite fields are deprioritised and appear below the favourited discussions we
+            // need to order the favourited discussions by id so that the most recently favourited discussions
+            // appear at the top of the list.
+            $favouritesort .= ", {$favalias}.id DESC";
+        }
 
         return "{$alias}.pinned DESC $favouritesort , {$keyfield} {$direction}";
     }

mod/forum/version.php

@@ -24,6 +24,6 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$plugin->version   = 2019040402;       // The current module version (Date: YYYYMMDDXX)
+$plugin->version   = 2019040403;       // The current module version (Date: YYYYMMDDXX)
 $plugin->requires  = 2018112800;       // Requires this Moodle version
 $plugin->component = 'mod_forum';      // Full name of the plugin (used for diagnostics)