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select_related

2024-10-27 21:05:39   211人阅读

作用:减少DB访问次数

from django.db import modelsclass Blog(models.Model):    name = models.CharField(max_length=100)    tagline = models.TextField()    def __str__(self):              # __unicode__ on Python 2        return self.nameclass Author(models.Model):    name = models.CharField(max_length=50)    email = models.EmailField()    def __str__(self):              # __unicode__ on Python 2        return self.nameclass Entry(models.Model):    blog = models.ForeignKey(Blog)    headline = models.CharField(max_length=255)    body_text = models.TextField()    pub_date = models.DateField()    mod_date = models.DateField()    authors = models.ManyToManyField(Author)    n_comments = models.IntegerField()    n_pingbacks = models.IntegerField()    rating = models.IntegerField()    def __str__(self):              # __unicode__ on Python 2        return self.headline

e = Entry.objects.get(id=5) //访问一次数据库

b = e.blog //再访问一次数据库

e = Entry.objects.select_related(‘blog‘).get(id=5) //访问一次数据库

b = e.blog //不访问数据库

加入queryset中,顺序不重要

Entry.objects.filter(pub_date__gt=timezone.now()).select_related(‘blog‘)

Entry.objects.selected_related(‘blog‘).filter(pub_date__gt=timezone.now())

from django.db import modelsclass City(models.Model):    # ...    passclass Person(models.Model):    # ...    hometown = models.ForeignKey(City)class Book(models.Model):    # ...    author = models.ForeignKey(Person)

 

b = Book.objects.select_related(‘author__hometown‘).get(id=4)p = b.author         # Doesn‘t hit the database.c = p.hometown       # Doesn‘t hit the database.b = Book.objects.get(id=4) # No select_related() in this example.p = b.author         # Hits the database.c = p.hometown       # Hits the database.

清空条件

without_relations = queryset.select_related(None)

多重查找

 select_related(‘foo‘, ‘bar‘)

控制查询缓存深度

b = Book.objects.select_related(depth=1).get(id=4)

select_related


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