Store layer relations in extra table.

src/Netzmacht/Contao/Leaflet/Model/MapModel.php

@@ -16,4 +16,16 @@ class MapModel extends \Model
 {
     protected static $strTable = 'tl_leaflet_map';
 
+    /**
+     * @return \Model\Collection
+     */
+    public function findLayers()
+    {
+        $query  = 'SELECT l.* FROM tl_leaflet_layer l LEFT JOIN tl_leaflet_map_layer m ON l.id = m.lid WHERE m.mid=?';
+        $result = \Database::getInstance()
+            ->prepare($query)
+            ->execute($this->id);
+
+        return \Model\Collection::createFromDbResult($result, 'tl_leaflet_layer');
+    }
 }