Django：在同一个一对多关系中具有多个联接的单个查询

Question

使用Django QuerySet API，如何在同一个两个表/模型之间执行多个联接？出于说明目的，请参阅以下未经测试的代码：

class DataPacket(models.Model): 
    time = models.DateTimeField(auto_now_add=True) 

class Field(models.Model): 
    packet = models.ForeignKey(DataPacket, models.CASCADE) 
    name = models.CharField(max_length=25) 
    value = models.FloatField() 

我想获取仅包含特定命名字段的数据包列表。我想是这样的：

pp = DataPacket.prefetch_related('field_set') 
result = [] 
for p in pp: 
    o = { 
     f.name: f.value 
     for f in p.field_set.all() 
     if f.name in ('latitude', 'longitude') 
    } 
    o['time'] = p.time 
    result.append(o) 

但是，这已经被证明非常低效的，因为我有数百至数千的数据包的工作有很多，除了纬度和经度领域我想等领域。

是否有一个Django QuerySet调用转换为执行两个内部联接从datapacket表到field表不同的行上的内部联接？我可以用原始的SQL这样做，如下（假设Django的应用程序名为myapp）（再次，为了说明未经测试的代码）：

from django.db import connection 

with connection.cursor() as cursor: 
    cursor.execute(''' 
     SELECT p.time AS time, f1.value AS lat, f2.value AS lon 
     FROM myapp_datapacket AS p 
     INNER JOIN myapp_field as f1 ON p.id = f1.packet_id 
     INNER JOIN myapp_field as f2 ON p.id = f2.packet_id 
     WHERE f1.name = 'latitude' AND f2.name = 'longitude' 
    ''') 
    result = list(cursor) 

但直觉告诉我不要使用低级别的DB API如果我不必这样做。支持这种做法的可能原因可能是我的SQL代码可能与Django支持的所有DBM不兼容，或者我觉得由于误解了SQL命令而导致数据库被贬低的风险比我误解Django API呼叫等。

Answer 1

尝试在django Performing raw SQL queries。以及select related in raw request。

预取和对原始查询：

from django.db.models.query import prefetch_related_objects 
raw_queryset = list(raw_queryset) 
prefetch_related_objects(raw_queryset, ['a_related_lookup', 
'another_related_lookup', ...]) 

你举的例子：使用原始的queryset prefetch_related

from django.db.models.query import prefetch_related_objects 
raw_DataPacket = list(DataPacket.objects.raw) 
pp = prefetch_related_objects(raw_DataPacket, ['field_set']) 

例子：

型号：

class Country: 
    name = CharField() 
class City: 
    country = models.ForeignKey(Country) 
    name = models.CharField() 

prefetch_relate d：从信息从这些来源提供

from django.db.models.query import prefetch_related_objects 
#raw querysets do not have len() 
#thats why we need to evaluate them to list 
cities = list(City.objects.raw("select * from city inner join country on city.country_id = country.id where name = 'london'")) 
prefetch_related_objects(cities, ['country']) 

答：djangoproject - performing raw queries | Related * Question | Google docs question