django - 在 Django 查询集上使用 iterator()

Question

我最近遇到了一些奇怪的行为，需要检查我的理解。

我在模型中使用了一个简单的过滤器，然后迭代结果。

例如

allbooks = Book.objects.filter(author='A.A. Milne')

for book in allbooks:
   do_something(book)

奇怪的是，它只返回了部分书籍列表。

但是，当使用相同的代码并使用 iterator() 时，这似乎运行良好。

IE。

for book in allbooks.iterator():
    do_something(book)

知道为什么吗？

p.s.我确实查看了 Django 文档，但看不到查询集将如何被缓存在其他任何地方......

iterator() Evaluates the QuerySet (by performing the query) and returns an iterator over the results. A QuerySet typically caches its results internally so that repeated evaluations do not result in additional queries; iterator() will instead read results directly, without doing any caching at the QuerySet level. For a QuerySet which returns a large number of objects, this often results in better performance and a significant reduction in memory

Note that using iterator() on a QuerySet which has already been evaluated will force it to evaluate again, repeating the query.

Answer 1

oddly, it was returning only a partial list of books.

这不是查询集必须如何工作的。遍历查询集应该为您提供数据库返回的每条记录。调试你的代码。你会发现错误，否则再次调试它。

checkin REPL 很容易。运行 manage.py shell :

from app.models import Model
for o in Model.objects.filter(fieldname="foo"): print o

#Let's see DB query
from django.db import connection
print(connection.queries)