MDL-78520 Add question bank data mapper API guide

docs/apis/plugintypes/qbank/data-mapper.md

@@ -0,0 +1,105 @@
+---
+title: Data mapper API for export and import
+tags:
+  - Plugins
+  - Question
+  - qbank
+description: The data mapper API allows you to export and import additional question data stored by your plugin.
+documentationDraft: true
+---
+
+If your question bank plugin stores additional data about a question, you can export and import that data along
+with the question by implementing the data mapper API. This is currently only supported by the Moodle XML export format.
+
+To implement a data mapper for your plugin, create a new class that extends `\core_question\local\bank\data_mapper_base`,
+and return an instance from `get_data_mapper` in your `plugin_feature` class:
+
+```php title="question/bank/example/classes/data_mapper.php"
+namespace qbank_example;
+
+class data_mapper extends \core_question\local\bank\data_mapper_base {
+
+}
+```
+
+```php title="question/bank/example/classes/plugin_feature.php"
+namespace qbank_example;
+
+class plugin_feature extends plugin_features_base {
+    #[\Override]
+    public function get_data_mapper(): data_mapper_base {
+        return new data_mapper();
+    }
+}
+```
+
+The data mapper API provides two methods, `get_question_data` to return the additional data related to each question
+for export, and `save_question_data` to take imported data and save it against an imported question.
+
+## get_question_data
+
+`get_question_data` must return an multi-dimensional array, keyed by the ID of the question that data belongs to.
+Even if the plugin has no data to export for a given question, it should return that key with an empty array. Starting
+your `get_question_data` with `$questiondata = parent::get_question_data($questionids);` will give you an empty array
+for each question ID.
+
+The resulting array should be in the format `$questiondata[$questionid][$itemidentifier] = [$key => $value]`, where
+`$itemidentifier` is some human-readable identifier for the data item you are exporting (such as shortname or idnumber),
+and the `[$key => $value]` array is a list of arbitrary data fields and values you want to export for that item.
+
+```php title="question/bank/example/classes/data_mapper.php"
+    #[\Override]
+    public function get_question_data(array $questionids): array {
+        global $DB;
+        $questiondata = parent::get_question_data($questionids);
+        foreach ($questionids as $questionid) {
+            $exampleitem = $DB->get_record('qbank_example', ['questionid' => $questionid]);
+            $questiondata[$questionid][$exampleitem->idnumber] = [
+                'field1' => $exampleitem->field1,
+                'field2' => $exampleitem->field2,
+           ];
+        }
+        return $questiondata;
+    }
+```
+
+## save_question_data
+
+`save_question_data` will recieve the ID of the question that has been imported, and an array of additional data
+for this plugin in from the imported data. This will match the format of the `$questiondata[$questionid]` array
+from `get_question_data`. You can then do whatever processing is required to store this data against the newly
+imported question.
+
+This function should return an array like `['error' => 'error messages', 'notice' => 'notice messages']` containing
+any messages that need to be reported from the import process, such as invalid data or non-existant fields.
+Starting your method with `$return = parent::save_question_data($questionid, $data);` will generate this array
+for you.
+
+```php title="question/bank/example/classes/data_mapper.php"
+    #[\Override]
+    public function save_question_data(int $questionid, array $data): array {
+        global $DB;
+        $return = parent::save_question_data($questionid, $data);
+        try {
+            $validfields = ['field1', 'field2'];
+            foreach ($data as $itemid => $itemdata) {
+                $example = [
+                    'questionid' => $questionid,
+                    'idnumber' => $itemid,
+                ];
+                foreach ($itemdata as $field => $value) {
+                    if (!in_array($field, $validfields)) {
+                        $return['notice'] .= "Skipped invalid field '{$field}' for question ID '{$questionid}'. ";
+                        continue;
+                    }
+                    $example[$field] = $value;
+                }
+                $DB->insert_record('qbank_example', $example);
+            }
+        } catch (\Throwable $e) {
+            $return['error'] = $e->getMessage();
+        }
+
+        return $questiondata;
+    }
+```

docs/apis/plugintypes/qbank/index.md

@@ -22,5 +22,6 @@ Question bank plugins can extend the question bank in many ways, including:
 - Bulk actions
 - Navigation node (tabs)
 - Question preview additions (via callback)
+- [Map additional question data for import/export](./data-mapper.md)
 
 The place to start implementing most of these is with a class `classes/plugin_features.php` in your plugin, that declares which features you want to add to the question bank. Until more documentation is written, looking at the examples of the plugins in Moodle core should give you a good idea what you need to do.