django-prefetch-decorator

Django 1.4 has prefetch_related, and it's grate! But sometimes it's hard to guess what we need to prefetch. And here comes prefetch decorator.

Example

Here's a simplified example of using prefetch decorator:

from django.conf import settings
from prefetch import prefetch
from dateutil import tz

settings.PREFETCH_ALIASES = {
    'region': 'this_app.Region',
    'business': 'this_app.Business'
}

class Timezone(model.Model):
    long_name = models.TextField()


class Region(models.Model):
    timezone = models.ForeignKey(Timezone, null=True)


    @prefetch(region=['timezone'])
    def getz(self):
        return tz.gettz(self.timezone.long_name)  # add timezone to region's prefetch


class Business(models.Model):
     regions = models.ManyToManyField('this_app.Region',
                                      related_name='businesses')

    # alternative @prefetch(business=[('regions', ('region', 'gettz'))])
    @prefetch(business=[('regions', ('this_app.Region', 'gettz'))])
    def get_timezone(self):
    try:
        r = self.regions.all()[0]  # small amount of regions (nice to prefetch)
        return r.gettz() # add this sentence to prefetch
    except IndexError:
        return tz.gettz('UTC')


def some_view(request):
    # (...)
    r = Business.objects.prefetch_related(Business.get_timezone.get_prefetch(Business)).get(pk=id)
    r.get_timezone()  # fully prefetch for this func no db calls



>>> Business.get_timezone.get_prefetch
{<function lib.prefetch.<lambda>>: [<function lib.prefetch.<lambda>>, 'regions']}

# after lazy loadings (when get_prefetch was called) same prefetch:
{some.path.to.model.Business: ['regions__timezone']}