Fix for broken fix about multi-fields in single line

Fix for error in jazzband#539

polymorphic/admin/childadmin.py

@@ -227,17 +227,18 @@ def get_subclass_fields(self, request, obj=None):
 
         # Find which fields are not part of the common fields.
         for fieldset in self.get_base_fieldsets(request, obj):
-            # multiple elements in single line
-            if isinstance(field, tuple):
-                for line_field in field:
+            for field in fieldset[1]["fields"]:
+                # multiple elements in single line
+                if isinstance(field, tuple):
+                    for line_field in field:
+                        try:
+                            subclass_fields.remove(line_field)
+                        except ValueError:
+                            pass  # field not found in form, Django will raise exception later.
+                else:
+                    # regular one-element-per-line
                     try:
-                        subclass_fields.remove(line_field)
+                        subclass_fields.remove(field)
                     except ValueError:
                         pass  # field not found in form, Django will raise exception later.
-            else:
-                # regular one-element-per-line
-                try:
-                    subclass_fields.remove(field)
-                except ValueError:
-                    pass  # field not found in form, Django will raise exception later.
         return subclass_fields