MDL-46647 grades: Fix fetch_all_helper() towards cross-db

That helper, used to fetch information from DB by all the grade_object
chidren classes was not behaving properly handling TEXT/CLOB columns.

Instead of creating a property within every class listing the
existing columns, it seems to be a better solution to instrospect
the database metadata (cached) to ensure the correct SQL is generated
in every case.

lib/grade/grade_object.php

@@ -177,6 +177,8 @@ protected static function fetch_helper($table, $classname, $params) {
      * @return array|bool Array of object instances or false if not found
      */
     public static function fetch_all_helper($table, $classname, $params) {
+        global $DB; // Need to introspect DB here.
+
         $instance = new $classname();
 
         $classvars = (array)$instance;
@@ -185,14 +187,25 @@ public static function fetch_all_helper($table, $classname, $params) {
         $wheresql = array();
         $newparams = array();
 
+        $columns = $DB->get_columns($table); // Cached, no worries.
+
         foreach ($params as $var=>$value) {
             if (!in_array($var, $instance->required_fields) and !array_key_exists($var, $instance->optional_fields)) {
                 continue;
             }
+            if (!array_key_exists($var, $columns)) { // TODO: Should this throw a coding exception?
+                continue;
+            }
             if (is_null($value)) {
                 $wheresql[] = " $var IS NULL ";
             } else {
-                $wheresql[] = " $var = ? ";
+                if ($columns[$var]->meta_type === 'X') {
+                    // We have a text/clob column, use the cross-db method for its comparison.
+                    $wheresql[] = ' ' . $DB->sql_compare_text($var) . ' = ' . $DB->sql_compare_text('?') . ' ';
+                } else {
+                    // Other columns (varchar, integers...).
+                    $wheresql[] = " $var = ? ";
+                }
                 $newparams[] = $value;
             }
         }