Question

12月29日：更新模型

我有三个模型如下：

class Job(models.Model):
    job_number = models.CharField(max_length=20, primary_key=True)

class Project(models.Model):
    job = models.ForeignKey(Job, null=True) # updated (null=True)***
    source = models.ForeignKey(Source) # added***

class Task(models.Model):
    project = models.ForeignKey(Project)

class Source(models.Model): # added***
    blahblah...

我想获得任务的工作号码。如下所示：

job = Job.objects.all().select_related()
jobno = job[0].project.job.job_number

我不确定上面的查询会在多长时间内触及数据库。但我猜它会超过两倍，不是吗？

select_related只能预先缓存2个表的外键到我的理解。在这种情况下，任何人都可以建议最佳做法来减少击中数据库的次数吗？

Answer 1

select_related()在一个查询中加入所有这三个模型：

>>> from app.models import Task
>>> task = Task.objects.all().select_related()[0]
>>> task.project.job.job_number
u'123'
>>> from django.db import connection
>>> len(connection.queries)
1
>>> connection.queries
[{u'time': u'0.002', u'sql': u'QUERY = u\'SELECT "app_task"."id", "app_task"."project_id", "app_project"."id", "app_project"."job_id", "app_job"."job_number" FROM "app_task" INNER JOIN "app_project" ON ( "app_task"."project_id" = "app_project"."id" ) INNER JOIN "app_job" ON ( "app_project"."job_id" = "app_job"."job_number" ) LIMIT 1\' - PARAMS = ()'}]
>>>

可读的SQL：

SELECT "app_task"."id", "app_task"."project_id", "app_project"."id",
       "app_project"."job_id", "app_job"."job_number"
FROM "app_task"
INNER JOIN "app_project" ON ( "app_task"."project_id" = "app_project"."id" )
INNER JOIN "app_job" ON ( "app_project"."job_id" = "app_job"."job_number" )

Answer 2

您可以使用过滤器：

task = Task.objects.all().select_related().filter(
    project__id__isnull=False, 
    job__id__isnull=False)

Django：如何使用select_related到INNER JOIN FK的FK

2 个答案: