[update] introduce Model::FIELD_FILTER_ONLY_FIELDS and use it

src/Model.php

@@ -83,6 +83,8 @@ class Model implements \IteratorAggregate
     public const FIELD_FILTER_EDITABLE = 'editable';
     /** @const string */
     public const FIELD_FILTER_PERSIST = 'persist';
+    /** @const string */
+    public const FIELD_FILTER_ONLY_FIELDS = 'only fields';
 
     // {{{ Properties of the class
 
@@ -811,16 +813,8 @@ public function get(string $field = null)
         if ($field === null) {
             // Collect list of eligible fields
             $data = [];
-            if ($this->only_fields) {
-                // collect data for actual fields
-                foreach ($this->only_fields as $field) {
-                    $data[$field] = $this->get($field);
-                }
-            } else {
-                // get all fields
-                foreach ($this->getFields() as $field => $f) {
-                    $data[$field] = $this->get($field);
-                }
+            foreach ($this->getFields(self::FIELD_FILTER_ONLY_FIELDS) as $field) {
+                $data[$field] = $this->get($field);
             }
 
             return $data;