开发笔记:Djangomodel进阶以及优化

querysey=models.Book.objects.all()
for book in querysey:
print(book.title)

Book.objects.all()[:3]
Book.objects.all()[3:6]
# 不支持负的索引，例如Book.objects.all()[0:-1]。通常，查询集的切片返回一个新的查询集，它不会执行查询。

queryset = models.Book.objects.all() # not hits database
print(queryset) # hits database
for book in queryset:
print(book.title) # Hits database

print(book.title for book in models.Book.objects.all()) # hits database
print(book.price for book in models.Book.objects.all()) # hits database

queryset = models.Book.objects.all()
print(book.title for book in queryset)
print(book.price for book in queryset)
# 下面也是一次数据库查询
for book in querysey:
print(book.title)
print(book.price)

queryset = models.Book.objects.all()
print(queryset[1]) # hits database
print(queryset[1]) # hits database

queryset = models.Book.objects.all()
[book for book in queryset] # hits database
print(queryset[1]) # use cache
print(queryset[1]) # use cache

queryset = models.Book.objects.all()
print(queryset) # hits database
print(queryset) # hits database

# iterator() 可以一次只从数据库获取少量数据，这样可以节省内存
objs = models.Book.objects.all().iterator()
for obj in objs:
print(obj.title)
# 注意，再次遍历没有打印，因为迭代器已经在上一次遍历（next）到最后一次了，没得遍历了
for obj in objs:
print(obj.title)