prefetch_related()¶
prefetch_related(*lookups)¶
Returns a QuerySet that will automatically retrieve, in a single batch, related objects for each of the specified lookups.
This has a similar purpose to select_related, in that both are designed to stop the deluge of database queries that is caused by accessing related objects, but the strategy is quite different.
select_related works by creating an SQL join and including the fields of the related object in the SELECT statement. For this reason, select_related gets the related objects in the same database query. However, to avoid the much larger result set that would result from joining across a 'many' relationship, select_related is limited to single-valued relationships - foreign key and one-to-one.
prefetch_related, on the other hand, does a separate lookup for each relationship, and does the 'joining' in Python. This allows it to prefetch many-to-many and many-to-one objects, which cannot be done using select_related, in addition to the foreign key and one-to-one relationships that are supported by select_related. It also supports prefetching of GenericRelation and GenericForeignKey, however, it must be restricted to a homogeneous set of results. For example, prefetching objects referenced by a GenericForeignKey is only supported if the query is restricted to one ContentType.
For example, suppose you have these models:
from django.db import models class Topping(models.Model): name = models.CharField(max_length=30) class Pizza(models.Model): name = models.CharField(max_length=50) toppings = models.ManyToManyField(Topping) def __str__(self): # __unicode__ on Python 2 return "%s (%s)" % ( self.name, ", ".join(topping.name for topping in self.toppings.all()), )
and run:
>>> Pizza.objects.all()
Url : https://docs.djangoproject.com/en/1.10/ref/models/querysets/
Dear All,--Kindly assist me to have a tutorial or code to make this kind of table in django. I failed to do this from the datatable .net tutorial.regards,--Eugene
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