使用https://docs.djangoproject.com/en/dev/topics/db/queries/#making-queries 中的模型并稍作修改:
from django.db import models
class Blog(models.Model):
name = models.CharField(max_length=100)
class Author(models.Model):
name = models.CharField(max_length=200)
joined = models.DateField()
def __str__(self):
return self.name
class Entry(models.Model):
blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
headline = models.CharField(max_length=255)
authors = models.ManyToManyField(Author)
rating = models.IntegerField()
我想创建一个从作者到条目的字典,作者今年加入,条目的评分为 4 或更高。结果字典的结构应如下所示:
author_entries = {author1: [set of entries], author2: [set of entries], etc.}
而访问数据库的次数少于 3 次(或至少与作者或条目的数量不成正比)。
我的第一次尝试(db hits == 作者数量,100 个作者 100 db-hits):
res = {}
authors = Author.objects.filter(joined__year=date.today().year)
for author in authors:
res[author] = set(author.entry_set.filter(rating__gte=4))
第二次尝试,尝试一次性阅读条目:
res = {}
authors = Author.objects.filter(joined__year=date.today().year)
entries = Entry.objects.select_related().filter(rating__gte=4, authors__in=authors)
for author in authors:
res[author] = {e for e in entries if e.authors.filter(pk=author.pk)}
这个更糟,100 个作者,198 个 db-hits(最初使用的第二次尝试{e for e in entries if author in e.authors},但 Django 没有它。
我发现的唯一方法涉及 raw-sql(4 db-hits):
res = {}
_authors = Author.objects.filter(joined__year=date.today().year)
_entries = Entry.objects.select_related().filter(rating__gte=4, authors__in=_authors)
authors = {a.id: a for a in _authors}
entries = {e.id: e for e in _entries}
c = connection.cursor()
c.execute("""
select entry_id, author_id
from sampleapp_entry_authors
where author_id in (%s)
""" % ','.join(str(v) for v in authors.keys()))
(对于在c.execute(..)调用中使用字符串替换表示歉意——我找不到where in ?调用所需的语法 sqlite )。
有没有更 Djangoesque 的方式来做到这一点?
我用我正在使用的代码(https://github.com/thebjorn/revm2m)创建了一个 git repo ,测试在https://github.com/thebjorn/revm2m/blob/master/revm2m/示例应用程序/tests.py
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