[英]Django: Single query with multiple joins on the same one-to-many relationship
使用Django QuerySet API,如何在相同的兩個表/模型之間執行多個聯接? 出於說明目的,請參見以下未經測試的代碼:
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()
我想獲取僅具有特定命名字段的數據包列表。 我嘗試過這樣的事情:
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)
但是事實證明這效率極低,因為我正在處理數百到數千個數據包,除了我想要的緯度和經度字段外,還有許多其他字段。
是否有Django QuerySet調用轉換為有效的SQL查詢,該查詢執行從數據datapacket
表到不同行的field
表的兩個內部聯接? 我可以使用原始SQL做到這一點,如下所示(假設Django應用程序名為myapp
)(再次,未經測試的代碼用於說明目的):
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)
但是本能告訴我,如果不需要,請不要使用低級數據庫API。 支持該問題的可能原因可能是我的SQL代碼可能與Django支持的所有DBM不兼容,或者我覺得我由於誤解SQL命令比誤解Django API更有可能破壞數據庫。電話等
嘗試在Django中執行原始SQL查詢 。 以及在原始請求中選擇相關 。
預取原始查詢:
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', ...])
你的例子:
from django.db.models.query import prefetch_related_objects
raw_DataPacket = list(DataPacket.objects.raw)
pp = prefetch_related_objects(raw_DataPacket, ['field_set'])
prefetch_related與原始查詢集的示例:
楷模:
class Country:
name = CharField()
class City:
country = models.ForeignKey(Country)
name = models.CharField()
prefetch_related:
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'])
來自以下來源的信息提供的答案: djangoproject-執行原始查詢 | 相關的Stackoverflow問題 | Google文檔問題
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