database - Django - 模型 save() 方法是懒惰的吗？

Question

Django 中的模型 save() 方法是惰性的吗？

例如，在下面的代码示例中，django 将在哪一行访问数据库？

my_model = MyModel()
my_model.name = 'Jeff Atwood'
my_model.save()
# Some code that is independent of my_model...
model_id = model_instance.id
print (model_id)

Answer 1

延迟保存没有多大意义，对吗？ Django 的 QuerySets是懒惰的，模型的 save 方法不是。

来自 django 源代码:

django/db/models/base.py，第 424–437 行:

def save(self, force_insert=False, force_update=False, using=None):
    """
    Saves the current instance. Override this in a subclass if you want to
    control the saving process.

    The 'force_insert' and 'force_update' parameters can be used to insist
    that the "save" must be an SQL insert or update (or equivalent for
    non-SQL backends), respectively. Normally, they should not be set.
    """
    if force_insert and force_update:
        raise ValueError("Cannot force both insert and updating in \
            model saving.")
    self.save_base(using=using, force_insert=force_insert, 
        force_update=force_update)

save.alters_data = True

然后，save_base 执行繁重的工作(同一文件，第 439–545 行):

...
transaction.commit_unless_managed(using=using)
...

在 django/db/transaction.py 的第 167–178 行中，您会发现:

def commit_unless_managed(using=None):
    """
    Commits changes if the system is not in managed transaction mode.
    """
    ...

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附言所有行号都适用于 django 版本 (1, 3, 0, 'alpha', 0)。