如何判斷一個django RelatedManager.add（）是否發現該對象已被添加？

Question

我有一個post_save功能

@receiver(post_save) 
def update_parent_location(sender, instance=None, created=False, **kwargs): 
list_of_models = ('A', 'B', 'C') 

if sender.__name__ in list_of_models: 
    groups = instance.group_set.all() 
    fields_to_update = tuple(field.lower() for field in list_of_models) 

    for field in fields_to_update: 
     parent = getattr(instance, field, None) 
     if parent is not None: 
      parent.groups.add(*groups) 
      parent.save() 

我怎麼會知道，如果

parent.groups.add(*groups) 

部分實際上增加了一個新的對象或默默失敗下面的代碼？

Answer 1

你可以查詢組之前和之後數，看看是否有什麼改變：

... 
num_groups = parent.groups.count() 
parent.groups.add(*groups) 
if parent.groups.count() > num_groups: 
    # Something was added 
    parent.save() 
...