What is the difference between .save () and .update () in Django?

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What is the difference between .save () and .update () in Django?

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#1 by (2 votes)
#2 by (2 votes)

2

I'm having trouble understanding the difference between them, since I realized that I was using the .save () method to change my values, but I noticed that .update () exists.

django

asked by anonymous 04.12.2015 / 01:29

2 answers

2

There are several differences between the two.

The Update can be used to update multiple objects, Save works to save a single row in the database.

In the case of multiple changes the Update will give you much more performance, because it is a call by queryset.

In contrast to Save is very easy to be written, as in the example below:

Class myModel(models.Model): 
    name = models.CharField()
    date_created = models.DateField()

    def save(self):
        if not self.pk :
           self.date_created = datetime.datetime.now()
        super(myModel , self).save()

It's nothing wrong with using Save , but if you have situations where you can make bulk changes, consider using Update

UPDATE

Save executes any overwriting, the Update will not execute overwriting of the save method of the model.

Here has a legal guide saying what exactly happens at the time of saving and also talks about how he knows whether it is an inclusion or change.

04.12.2015 / 02:16

How can I find out which packages / libraries belong to the functions of a routine written in R? "order by clauses" can create conflicts with each other?

score 2 · Accepted Answer

The save method saves changes to a single object in the model (that is, a single DB line), whereas update can change multiple objects at the same time. The differences are as follows:

objeto.save()

Is called in a single object;
Can be overwritten in the model to execute proper logic before or after saving in fact;
Can be used to create a new object or update an existing object. It chooses between one and another depending on whether the object already exists in the DB or not.
- To ensure that Django only saves if the object does not exist, pass the force_insert=True argument to the save method. It throws an exception if the object already exists, instead of making a UPDATE .
- To ensure that Django only saves if the object exists, pass the force_update=True argument to the save method. It throws an exception if the object does not exist, instead of doing a INSERT .

queryset.update()

It is called in a queryset , which can affect zero or more objects simultaneously;
- It is even called before of queryset be evaluated, so it does not SELECT and then UPDATE - it only does UPDATE .
Does not allow you to run your own logic before or after saving each individual object. In particular, it does not call the% method of% of each updated object;
- If you need some custom logic during the save, do not use the save method. Instead save in a loop:
```
for objeto in queryset:
    objeto.save()
```
  This is less efficient than the update method (because it does N + 1 SQL commands instead of just 1), but may be necessary in some situations.
Updates all objects the queryset to a single, same value in a single SQL command. If the final value can be calculated from the current values of the records, you can use expressions F along with this command. Example:
```
MeuModelo.objects.filter(campo1=42).update(campo2 = F('campo2') + 1)
```
(Increments in 1 column update of the entire line that has campo2 % equal to 42, all in a single SQL command)