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例子：

a.b_id # 正确

a.b.id  # 错误

比如：a表存在外键b表

a.b.all() # 错误

values返回的是字典数组，比如：[{'key1': value1, 'key2': value2}, {'key1': value3, 'key2': value4}]

value_list返回的是tuple数组 [('value1', 'value2'), ('value3', 'value4')]

value_list+flat=True返回的是数组 ['value1', ...]

#  Don't waste a query if you are using the queryset

books = Book.objects.filter(..)

if len(books) > 5:

  do_stuff_with_books(books)

#  If you aren't using the queryset use count

books = Book.objects.filter(..)

if books.count() > 5:

  do_some_stuff()

#  But never

if len(Book.objects.filter(..)) > 5:

  do_some_stuff()

比如:

for book in Books.objects.all().iterator():

    do_stuff(book)