Django – 模型的save()方法是否延迟执行

在本文中，我们将介绍Django模型中的save()方法是否延迟执行。save()方法是Django模型中一个重要的函数，用于保存数据到数据库中。

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什么是save()方法？

save()方法是Django模型中的一个内置方法，用于将模型的数据保存到数据库中。当我们在创建了一个新的对象实例并对其进行修改后，我们可以调用save()方法将其保存到数据库中。在save()方法被调用之前，模型实例并没有被保存到数据库中，而是保存在内存中。

save()方法的延迟执行

Django的save()方法是延迟执行的，这意味着在调用save()方法之前，对模型实例的所有更改都不会立即生效。相反，Django会将这些更改存储在一个队列中，并在适当的时候批量将更改应用到数据库中。

为了更好地理解save()方法的延迟执行，我们来看一个示例。假设我们有一个User模型，其中包含name和email两个字段。

class User(models.Model):
    name = models.CharField(max_length=100)
    email = models.EmailField()

    def save(self, *args, **kwargs):
        # 在保存之前对name字段进行修改
        self.name = self.name.upper()
        super().save(*args, **kwargs)

在上面的示例中，我们在模型的save()方法中将name字段转换为大写，并且调用了父类的save()方法。当我们创建一个User实例并调用save()方法时，Django并不会立即将更改应用到数据库中。相反，Django会将这个修改操作添加到保存队列中，直到适当的时机才会将所有修改应用到数据库中。

什么时候会执行save()方法？

在Django中，save()方法可以在多种情况下被自动调用，比如当我们调用模型的create()方法创建新的对象实例时，当我们调用模型的update()方法更新对象实例时，当我们通过模型表单保存数据时等等。

下面是一些常见的情况，Django会自动调用save()方法：

• 当我们调用模型的create()方法创建新的对象实例时，Django会在保存对象之前调用save()方法。

user = User.objects.create(name='John', email='john@example.com') # 调用save()方法将user保存到数据库

• 当我们调用模型的update()方法更新对象实例时，Django会在保存更新后的对象之前调用save()方法。

user = User.objects.get(name='John')
user.name = 'John Doe'
user.save() # 调用save()方法将更新后的user保存到数据库

• 当我们通过模型表单保存数据时，Django会自动调用save()方法将表单中的数据保存到数据库。

def save_user(request):
    if request.method == 'POST':
        form = UserForm(request.POST)
        if form.is_valid():
            form.save() # 调用save()方法将表单数据保存到数据库

• 当我们直接调用模型实例的save()方法时，Django会将实例的更改保存到数据库。

user = User.objects.get(name='John')
user.name = 'John Doe'
user.save() # 调用save()方法将更新后的user保存到数据库

需要注意的是，如果我们对一个模型实例进行了修改但没有调用save()方法保存，那么这些修改将不会生效，也不会被保存到数据库中。

save()方法的调用时机和性能优化

如前所述，Django的save()方法是延迟执行的。而且，在一次请求中，所有对模型的更改都会在请求结束时批量保存到数据库中。这种设计有助于提高性能，因为将多个save()操作合并为一个批量操作可以减少数据库交互的次数，降低了开销。

然而，有时我们可能希望在调用save()方法后立即将更改应用到数据库中，而不是等到请求结束时。在这种情况下，我们可以使用Django的force_insert和force_update参数来强制执行save()方法。

• 使用force_insert参数可以强制将save()方法作为插入操作执行，而不管对象是否已经存在于数据库中。

user = User(name='John', email='john@example.com')
user.save(force_insert=True) # 强制将save()方法作为插入操作执行

• 使用force_update参数可以强制将save()方法作为更新操作执行，而不管对象是否已经存在于数据库中。

user = User.objects.get(name='John')
user.name = 'John Doe'
user.save(force_update=True) # 强制将save()方法作为更新操作执行

总结

在本文中，我们讨论了Django模型中的save()方法是否延迟执行。我们了解到，Django的save()方法是延迟执行的，这意味着在调用save()方法之前，对模型的更改不会立即生效，而是保存在内存中，直到适当的时机才会批量应用到数据库中。

我们还了解了save()方法的调用时机，包括在调用create()、update()方法时以及使用模型表单保存数据时等等。同时，我们探讨了如何通过force_insert和force_update参数来强制执行save()方法，以便立即将更改应用到数据库中。

通过深入了解Django模型的save()方法的延迟执行，我们可以更好地掌握Django的工作原理，并在需要的时候进行性能优化。

Django – Are model save() methods lazy?

In this article, we will explore whether the save() methods in Django models are lazy. The save() method is an essential function in Django models used to save data into the database.

What is the save() method?

The save() method is an inbuilt method in Django models that is used to save the model’s data into the database. When we create a new object instance and modify it, we can call the save() method to save it into the database. Before the save() method is called, the model instance is not saved in the database but stored in memory.

Lazy execution of the save() method

The save() method in Django is lazy-executed, which means that the changes made to the model instance are not immediately applied until the save() method is called. Instead, Django stores these changes in a queue and applies them to the database in batches at the appropriate time.

To better understand the lazy execution of the save() method, let’s look at an example. Suppose we have a User model with two fields: name and email.

class User(models.Model):
    name = models.CharField(max_length=100)
    email = models.EmailField()

    def save(self, *args, **kwargs):
        # Modify the name field before saving
        self.name = self.name.upper()
        super().save(*args, **kwargs)

In theabove example, we are converting the name field to uppercase in the save() method of the model and then calling the save() method of the parent class. When we create a User instance and call the save() method, Django does not immediately apply the change to the database. Instead, it adds this modification operation to the save queue until it is appropriate to apply all the changes to the database.

When does the save() method get executed?

In Django, the save() method can be automatically invoked in various scenarios, such as when we call the create() method of the model to create a new object instance, when we call the update() method to update an object instance, or when we save data through a model form.

Here are some common scenarios where Django automatically calls the save() method:

• When we call the create() method of the model to create a new object instance, Django invokes the save() method before saving the object to the database.

user = User.objects.create(name='John', email='john@example.com') # The save() method is called to save the user to the database

• When we call the update() method of the model to update an object instance, Django invokes the save() method before saving the updated object to the database.

user = User.objects.get(name='John')
user.name = 'John Doe'
user.save() # The save() method is called to save the updated user to the database

• When we save data through a model form, Django automatically calls the save() method to save the form data to the database.

def save_user(request):
    if request.method == 'POST':
        form = UserForm(request.POST)
        if form.is_valid():
            form.save() # The save() method is called to save the form data to the database

• When we directly call the save() method of a model instance, Django saves the changes to the database.

user = User.objects.get(name='John')
user.name = 'John Doe'
user.save() # The save() method is called to save the updated user to the database

It’s important to note that if we make changes to a model instance but do not call the save() method, these modifications will not take effect and will not be saved to the database.

The timing of save() method invocation and performance optimization

As mentioned earlier, the save() method in Django is lazily executed. Furthermore, within a single request, all changes to the models are batched and applied to the database when the request is completed. This design helps improve performance by reducing the number of database interactions and overhead through batching multiple save() operations into a single one.

However, sometimes we may want to apply the changes to the database immediately after calling the save() method, rather than waiting until the end of the request. In such cases, we can use the force_insert and force_update parameters provided by Django to force the immediate execution of the save() method.

• By using the force_insert parameter, we can force the save() method to act as an insert operation regardless of whether the object already exists in the database.

user = User(name='John', email='john@example.com')
user.save(force_insert=True) # Force saving as an insert operation

• By using the force_update parameter, we can force the save() method to act as an update operation regardless of whether the object already exists in the database.

user = User.objects.get(name='John')
user.name = 'John Doe'
user.save(force_update=True) # Force saving as an update operation

Summary

In this article, we discussed whether the save() method in Django models is lazily executed. We learned that the save() method in Django is lazily executed, which means that the changes made to the model instance are not immediately applied and saved to the database until the save() method is called.

We also explored the timing of save() method invocation, including situations where it is automatically called such as when using create(), update(), or model forms. Moreover, we discussed how to use the force_insert and force_update parameters to force the immediate execution of the save() method to apply changes to the database.

By gaining a deeper understanding of the lazy execution of the save() method in Django models, we can better grasp the workings of Django and optimize performance when needed.