[FIX] onchange_helper: compute related values for onchange applications

onchange_helper/models/base.py

@@ -51,13 +51,16 @@ class Base(models.AbstractModel):
         else:
             # We get default values, they may be used in onchange
             record_values = self.default_get(self._fields.keys())
-        for field in self._fields:
-            if field not in all_values:
-                all_values[field] = record_values.get(field, False)
+
+        for fn, field in self._fields.items():
+            if not field.related and fn not in all_values:  # related fields are False
+                all_values[fn] = record_values.get(fn, False)
 
         new_values = {}
+        with self.env.do_in_onchange():  # avoid cache issues with related fields!
+            new = self.new(all_values)
             for field in onchange_fields:
-            onchange_values = self.onchange(all_values, field, onchange_specs)
+                onchange_values = new.onchange(all_values, field, onchange_specs)
                 new_values.update(self._get_new_values(values, onchange_values))
                 all_values.update(new_values)