使用以下django模型:
class Author(models.Model):
name = models.CharField(max_length=100)
age = models.IntegerField()
class Book(models.Model):
name = models.CharField(max_length=300)
author = models.ForeignKey(Author)
pubdate = models.DateField()
class Meta:
ordering = ('-pubdate')
我怎样才能获得每位作者发表的五本最新书籍?
我曾考虑过对每个作者进行迭代,并将作者发表的书籍切成5个。
for a in Author.objects.all():
books = Book.objects.filter(author = a )[:5]
print books #and/or process the entries...
但是,如果这些表有很多记录(可能有数千本书),这可能会很慢且效率低下。
那么,还有其他方法可以用django(或sql查询)来实现这个目的吗?
答案 0 :(得分:2)
我建议:
for a in Author.objects.all():
books = a.book_set.all().order_by('-pub_date')[:5]
print books #and/or process the entries...
或者,如果订单应始终相同,则定义Meta,
books = a.book_set.all()[:5]
应该做的伎俩
答案 1 :(得分:1)
如果您担心查询的速度,请在pubdate字段上构建索引:
pubdate = models.DateField(db_index=True)
每次运行查询时都应避免扫描整个表。
postgres中的SQL类似于:
select b1.name, b1.author
from books b1
where b1.id in (
select b2.id
from books b2
where b1.author = b2.author
order by b2.pubdate desc
limit 3)
order by b1.author, b1.name