通过中间模型获取对象的查询集

Question

我想获取与某个content_object相关的所有Geom对象（请参阅我在底部尝试构建的函数，get_geoms_for_obj（）

class Geom(models.Model):
    ...

class GeomRelation(models.Model):
    ''' For tagging many objects to a Geom object and vice-versa'''

    geom = models.ForeignKey(Geom)
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey()

def get_geoms_for_object(obj):
    ''' takes an object and gets the geoms that are related

    '''
    ct = ContentType.objects.get_for_model(obj)
    id = obj.id
    grs = GeomRelation.objects.filter( content_type=ct, object_id=id )
    # how with django orm magic can I build the queryset instead of list
    # like below to get all of the Geom objects for a given content_object
    geoms = []
    for gr in grs:
        geoms.append(gr.geom)
    return set(geoms)
    # A set makes it so that I have no redundant entries but I want the
    # queryset ordering too .. need to make it a queryset for so many reasons...

Answer 1

咄，

return Geom.objects.filter(geomrelation__in=grs)

Answer 2

如果您的Geom类具有generic.GenericRelation（）属性，则可以通过标准向后关系获取对象。

class Geom(models.Model):
    ...
    geom_relations = generic.GenericRelation(GeomRelation)

这是仅限Python的属性，不需要更改数据库。现在要获取geom_relations，您只需：

geom.geom_relations.all()

Answer 3

如果将一个QuerySet对象作为另一个QuerySet对象的过滤器参数传递，则ORM将使用嵌套的select语句。最好在过滤器查找中桥接表关系;那么ORM将在JOIN上使用一个简单的WHERE子句，无论如何它必须做。性能差异很大：

In [2]: from django.db import connection
In [3]: from app.models import *
In [4]: ct = ContentType.objects.get_for_model(Thing)
In [5]: grs = GeomRelation.objects.filter( content_type=ct, object_id=2 )
In [6]: 
In [7]: # slow method
In [8]: list(Geom.objects.filter(geomrelation__in=grs));
In [9]: connection.queries[-1]
Out[9]: 
{'sql': u'SELECT "app_geom"."id" FROM "app_geom" INNER JOIN "app_geomrelation" ON ("app_geom"."id" = "app_geomrelation"."geom_id") WHERE "app_geomrelation"."id" IN (SELECT U0."id" FROM "app_geomrelation" U0 WHERE (U0."content_type_id" = 10  AND U0."object_id" = 2 ))',
 'time': '0.140'}
In [10]: 
In [11]: # fast method
In [12]: list(Geom.objects.filter(geomrelation__content_type=ct,
   ....:                         geomrelation__object_id=2));
In [13]: connection.queries[-1]
Out[13]: 
{'sql': u'SELECT "app_geom"."id" FROM "app_geom" INNER JOIN "app_geomrelation" ON ("app_geom"."id" = "app_geomrelation"."geom_id") WHERE ("app_geomrelation"."object_id" = 2  AND "app_geomrelation"."content_type_id" = 10 )',
 'time': '0.001'}