python Django：select_related 与 ManyToManyField
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Django : select_related with ManyToManyField
提问by Paul Tarjan
I have :
我有 ：
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 Aliaswhich has all its achiever_setand awardsprepopulated?
怎样才可以有一个Alias具有其所有achiever_set，并awards预填？
>>> 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?
我将需要大量访问别名已经获得的奖励（中间表和奖励本身）。我怎样才能批量处理所有这些？
回答by Vebjorn Ljosa
Django versions 1.4 and above have prefetch_relatedfor this purpose.
Django 1.4 及以上版本已prefetch_related用于此目的。
The prefetch_relatedmethod is similar to select_related, but does not do a database join. Instead, it executes additional database queries and does the joining in Python.
该prefetch_related方法类似于select_related，但不执行数据库连接。相反，它执行额外的数据库查询并在 Python 中进行连接。
回答by Shane
If you're not on Django 1.4, there's also the django-batch-selectlibrary, which works basically the same way as prefetch_related.
如果您使用的不是 Django 1.4，还有django-batch-select库，它的工作方式与 prefetch_related 基本相同。
