Django数据库查询操作
1、查询所有记录：模型名.objects.all()
>>> from django.contrib.auth.models import User>>> users = User.objects.all()>>> users<QuerySet [<User: admin>, <User: zhanglaoshi>, <User: wanglaoshi>, <User: zhaolaoshi>,<User: zhoulaoshi>, <User: wulaoshi>, <User: danglaoshi>, <User: canglaoshi>, <User:hulaoshi>, <User: yinlaoshi>, <User: lulaoshi>, <User: chenlaoshi>, <User: kuanglaoshi>,<User: xiaodou>, <User: xiaojin>]>查询结果保留在users序列中，获得序列中的一个元素，使用下述方式：
>>> lu = users[10]>>> lu<User: lulaoshi>>>> lu.email'lu@laoshi.com'2、精确查询：模型名.objects.get(字段名=字段值)
括号里面的是查询条件，可以设置一个以上的查询条件，中间用‘，’分开。
查询不到结果时：报错。
>>> BlogArticles.objects.get(id=1)<BlogArticles: php is the best computer language>>>> BlogArticles.objects.get(title="Canglaoshi is a good teacher")<BlogArticles: Canglaoshi is a good teacher>>>> BlogArticles.objects.get(title="teacher")Traceback (most recent call last):File "/usr/lib/python3.5/code.py", line 91, in runcodeexec(code, self.locals)File "<console>", line 1, in <module>File "/usr/local/lib/python3.5/dist-packages/django/db/models/manager.py", line 85,in manager_methodreturn getattr(self.get_queryset(), name)(*args, **kwargs)File "/usr/local/lib/python3.5/dist-packages/django/db/models/query.py", line 385, ingetself.model._meta.object_nameblog.models.DoesNotExist: BlogArticles matching query does not exist.3、根据条件查询：模型名.objects.filter(过滤条件)
过滤不到结果时：返回空。
>>> users_laoshi = User.objects.filter(username__endswith="laoshi")>>> users_laoshi<QuerySet [<User: zhanglaoshi>, <User: wanglaoshi>, <User: zhaolaoshi>, <User:zhoulaoshi>, <User: wulaoshi>, <User: danglaoshi>, <User: canglaoshi>, <User: hulaoshi>,<User: yinlaoshi>, <User: lulaoshi>, <User: chenlaoshi>, <User: kuanglaoshi>]>这里查询出username中以“laoshi”这个字符串结尾的对象。
除filter()能够实现根据条件查询外，还有exclude()
>>> users = User.objects.filter(username__endswith="laoshi").exclude(id__gt=7)>>> users<QuerySet [<User: zhanglaoshi>, <User: wanglaoshi>, <User: zhaolaoshi>, <User:zhoulaoshi>, <User: wulaoshi>, <User: danglaoshi>]>4、查询结果排序
对于查询返回的QuerySet结果，默认是按照数据模型类中定义的字段排序的，如果要重新排序，可以使用order_by()方法。
>>> u = User.objects.filter(username__startswith="z").order_by("username")>>> u<QuerySet [<User: zhanglaoshi>, <User: zhaolaoshi>, <User: zhoulaoshi>]>>>> u = User.objects.filter(username__startswith="z").order_by("-username")>>> u<QuerySet [<User: zhoulaoshi>, <User: zhaolaoshi>, <User: zhanglaoshi>]>order_by()的参数中可以写多个字段名称，即按照字段前后顺序分别作为排序的“主关键词”和“次关键词”。
5、使用外键查询
定义ArticleColumn模型时，定义外键user如下：
user = models.ForeignKey(User, on_delete=models.CASCADE,related_name='article_column')说明User一条记录，可以对应多个ArticleColumn记录，是一对多的关系，并且通过user可以查到其对应的ArticleColumn记录。related_name='article_columns' 提供了从 User 实例访问关联的 ArticleColumn 实例的方便方式。
user = User.objects.get(username='john_doe')
columns = user.article_columns.all() # 获取所有与这个用户关联的 ArticleColumn 实例
如果没有定义related_name，则使用下述代码查询相关的ArticleColumn记录。
user = User.objects.get(username='john_doe')
columns = user.articlecolumn_set.all() # 使用默认的反向关系名称
articlecolumn_set中的articlecolumn是模型名。