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

Question

I’m having trouble understanding the difference between them, because I realized I was using the method. save() to change my values, but I noticed that there is . update().

Answer 1

There are several differences between the two.

The Update can be used to update various objects, the Save works to save a single line in the bank.

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

Against the background of Save is very easy to be on writing, 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()

There’s nothing wrong with wearing the Save, but if you have situations where you can make mass changes think about using the Update

UPDATING

Save performs any over writing, Update will not run the over writing of the model’s Save method.

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

Answer 2

The method save saves changes to a single model object (i.e., a single BD line), while the update can change multiple objects at the same time. The differences are as follows:

1. objeto.save()

   • It is called in a single object;
   • Can be overwritten in the template to perform proper logic before or after actually saving;
   • It can be used to create a new object or update an existing object. It chooses between one and the other depending on whether the object already exists in the BD or not.
     • To ensure that Django only saves if the object does not exist, pass the argument force_insert=True to the method save. He will make an exception if the object already exists, instead of making a UPDATE.
     • To ensure that Django only saves if the object exists, pass the argument force_update=True to the method save. He will make an exception if the object does not exist instead of making a INSERT.

2. queryset.update()

   • It is called in a queryset, can affect zero or more objects simultaneously;
     • He’s even called before of queryset be evaluated, so that it does not make a SELECT and then a UPDATE - he just does the UPDATE.

   • It does not allow to execute logic of its own before or after saving each individual object. In particular, it flame the method save of each updated object;

     • If you need some custom logic while saving, don’t use the method update. Instead save in a loop:

       for objeto in queryset:
           objeto.save()
       

       This is less efficient than the method update (because it makes N+1 SQL commands instead of just 1), but may be necessary in some situations.

   • Updates all the objects of queryset for a single, same value, in a single SQL command. If the final value can be calculated from the current values of the records, one can use expressions F along with this command. Example:

     MeuModelo.objects.filter(campo1=42).update(campo2 = F('campo2') + 1)
     

     (Increment by 1 column campo2 of every line that has campo1 equal to 42, all this in a single SQL command)