Django : select_related with ManyToManyField

问题

I have :

class Award(models.Model) :
    name = models.CharField(max_length=100, db_index=True)

class Alias(models.Model) :
    awards = models.ManyToManyField('Award', through='Achiever')

class Achiever(models.Model):
    award = models.ForeignKey(Award)
    alias = models.ForeignKey(Alias)
    count = models.IntegerField(default=1)

How can I have an Alias which has all its achiever_set and awards prepopulated?

>>> db.reset_queries()
>>> Alias.objects.filter(id="450867").select_related("achiever_set__award").get().achiever_set.all()[0].award.name
u'Perma-Peddle'
>>> len(db.connection.queries)
3
>>> db.reset_queries()
>>> Alias.objects.filter(id="450867").select_related("awards").get().awards.all()[0].name
u'Dwarfageddon (10 player)'
>>> len(db.connection.queries)
2

I'm going to need a lot of access to the award that an alias has already gotten (both the intermediate table and the awards themselves). How can I batch all of these?

回答1:

Django versions 1.4 and above have prefetch_related for this purpose.

The prefetch_related method is similar to select_related, but does not do a database join. Instead, it executes additional database queries and does the joining in Python.

回答2:

If you're not on Django 1.4, there's also the django-batch-select library, which works basically the same way as prefetch_related.

来源：https://stackoverflow.com/questions/1387044/django-select-related-with-manytomanyfield