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我在Django的prefetch_related时遇到了麻烦。Django prefetch_related在很多领域总是产生空阵列
这是我的模特。
class Terrain(models.Model):
name = models.CharField(max_length=255)
class Route(models.Model):
name = models.CharField(max_length=255)
terrains = models.ManyToManyField(Terrain)
而这里的时候我用prefetch_related会发生什么。
>>> Route.objects.all()[0].terrains.all()
[<Terrain: Beach>]
>>> Route.objects.all().prefetch_related('terrains')[0].terrains.all()
[]
任何想法我在做什么错在这里?
Python版本2.7 Django的1.4版本 DB后端:SQLite的(我也试图与MySQL的,相同的结果)
下面是(从IPython的控制台)一些查询调试所以你可以看到发生了什么事情在那里。
In [11]: Route.objects.all().prefetch_related('terrains')[0].terrains.all()
DEBUG 2012-06-22 20:10:09,569 util (0.000) SELECT "asdf_route"."id", "asdf_route"."name" FROM "asdf_route" LIMIT 1; args=()
DEBUG 2012-06-22 20:10:09,571 util (0.000) SELECT ("asdf_route_terrains"."route_id") AS "_prefetch_related_val", "asdf_terrain"."id", "asdf_terrain"."name" FROM "asdf_terrain" INNER JOIN "asdf_route_terrains" ON ("asdf_terrain"."id" = "asdf_route_terrains"."terrain_id") WHERE "asdf_route_terrains"."route_id" IN (1); args=(1,)
Out[11]: []