在Django和Postgres中为select_for_update生成的查询序列的差异

Question

我面临一个奇怪的情况,在使用select_for_update()内部transaction.atomic()块时,Django和Postgres中记录的查询序列是不同的.

基本上我是一个ModelForm我正在验证cleaned_data数据库的重复请求的地方.然后在创建视图的form_valid()方法中,我正在保存实例.要在同一个事务中同时进行操作,我重写post()方法,并将这两个方法调用包装在里面transaction.atomic().

这是我上面所说的代码:

# Form
class MenuForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        user_id = kwargs.pop('user_id', None)
        super(MenuForm, self).__init__(*args, **kwargs)

    def clean(self):
        cleaned_data = super(MenuForm, self).clean()
        dish_name = cleaned_data.get('dish_name')
        menus = Menu.objects.select_for_update().filter(user_id=self.user_id)

        for menu in menus:
            if menu.dish_name == dish_name:
                self.add_error('dish_name', 'Dish already exists')
                return cleaned_data
        return cleaned_data

# CreateView
class MenuCreateView(CreateView):
    form_class = MenuForm

    def get_form_kwargs(self):
        kwargs = super(MenuCreateView, self).get_form_kwargs()
        kwargs.update({'user_id': self.request.session.get('user_id')})
        return kwargs

    def form_valid(self, form):
        user = User.objects.get(id=self.request.session.get('user_id'))
        form.instance.user = user
        return super(MenuCreateView, self).form_valid(form)

    def post(self, request, *args, **kwargs):
        form = self.get_form()

        with transaction.atomic():
            if form.is_valid():
                return self.form_valid(form)
            else:
                return self.form_invalid(form)

现在假设我同时发出两个请求,创建一个菜单相同的菜单.我希望第二个请求失败.但是,他们俩都在过世.看起来,第二个事务没有看到先前事务中完成的更改.因此,在menus返回的事务中,总数保持不变select_for_update().

鉴于Postgres默认的隔离级别是READ COMMITTED,我希望这些更改是可见的.所以,我尝试记录查询以查看COMMIT; 在正确的时间被解雇.这是django和postgres的查询日志:

Django日志:

SELECT "menu"."id", "menu"."dish_id", "menu"."dish_name" FROM "menu" WHERE ("menu"."dish_name" = "Test Dish") FOR UPDATE; args=("Test Dish")    
INSERT INTO "menu" ("dish_id", "dish_name") VALUES (2, "Test Dish") RETURNING "menu"."id"; args=(2, "Test Dish")
SELECT "menu"."id", "menu"."dish_id", "menu"."dish_name" FROM "menu" WHERE ("menu"."dish_name" = "Test Dish") FOR UPDATE; args=("Test Dish")
INSERT INTO "menu" ("dish_id", "dish_name") VALUES (2, "Test Dish") RETURNING "menu"."id"; args=(2, "Test Dish")

Postgres日志:

<2016-03-18 17:55:46.176 IST  0  2/31  56ebf3ca.aac0>LOG:  statement: SHOW default_transaction_isolation
<2016-03-18 17:55:46.177 IST  0  2/32  56ebf3ca.aac0>LOG:  statement: SET TIME ZONE 'UTC'
<2016-03-18 17:55:46.178 IST  0  2/33  56ebf3ca.aac0>LOG:  statement: SELECT t.oid, typarray
    FROM pg_type t JOIN pg_namespace ns
        ON typnamespace = ns.oid
    WHERE typname = 'hstore';

<2016-03-18 17:55:46.182 IST  0  2/34  56ebf3ca.aac0>LOG:  statement: BEGIN
<2016-03-18 17:55:46.301 IST  0  3/2  56ebf3ca.aac1>LOG:  statement: SHOW default_transaction_isolation
<2016-03-18 17:55:46.302 IST  0  3/3  56ebf3ca.aac1>LOG:  statement: SET TIME ZONE 'UTC'
<2016-03-18 17:55:46.302 IST  0  3/4  56ebf3ca.aac1>LOG:  statement: SELECT t.oid, typarray
    FROM pg_type t JOIN pg_namespace ns
        ON typnamespace = ns.oid
    WHERE typname = 'hstore';

<2016-03-18 17:55:46.312 IST  0  3/5  56ebf3ca.aac1>LOG:  statement: BEGIN
<2016-03-18 17:55:46.963 IST  0  3/5  56ebf3ca.aac1>LOG:  statement: SELECT "menu"."id", "menu"."dish_id", "menu"."dish_name" FROM "menu" 
WHERE ("menu"."dish_name" = "Test Dish") FOR UPDATE
<2016-03-18 17:55:46.964 IST  0  2/34  56ebf3ca.aac0>LOG:  statement: SELECT "menu"."id", "menu"."dish_id", "menu"."dish_name" FROM "menu" 
WHERE ("menu"."dish_name" = "Test Dish") FOR UPDATE
<2016-03-18 17:55:47.040 IST  23712  3/5  56ebf3ca.aac1>LOG:  statement: INSERT INTO "menu" ("dish_id", "dish_name") VALUES (2, "Test Dish")RETURNING "menu"."id"
<2016-03-18 17:55:47.061 IST  23712  3/5  56ebf3ca.aac1>LOG:  statement: COMMIT
<2016-03-18 17:55:47.229 IST  23713  2/34  56ebf3ca.aac0>LOG:  statement: INSERT INTO "menu" ("dish_id", "dish_name") VALUES (2, "Test Dish")RETURNING "menu"."id"
<2016-03-18 17:55:47.231 IST  23713  2/34  56ebf3ca.aac0>LOG:  statement: COMMIT

Postgres.conf:

max_connections = 100
log_destination = 'stderr'
logging_collector = on
log_directory = 'pg_log'
log_line_prefix = '<%m  %x  %v  %c>'
log_statement = 'all'

如您所见,SELECT和INSERT查询的顺序在两个日志中都不相同.我无法理解为什么会这样.另外,如果您注意到,Postgres日志中SELECT查询的session_id也不同.这可以解释一下吗？

如果这是预期的行为,我怎么能在这里解决核心问题？根据现有记录避免并发INSERT查询.

更新:

我没有提到忽略重复菜单的实际逻辑不仅仅基于菜名.上面的一个是简化的例子.

将菜单模型视为:

class Menu:
    user_id = models.IntegerField()
    dish = models.ForeignKey(Dish)
    order_start_time = models.DateTimeField()
    order_end_time = models.DateTimeField()

实际逻辑是这样的:

• 使用dish_namefrom db 获取所有菜单.
• 检查order_start_time和order_end_time所有的菜单,看看其中是否与重叠order_start_time,并order_end_time为新菜单.如果发现冲突,请避免添加.

所以,我们可以为菜添加两个菜单 - d1有订单窗口 - [9am-10am]和[2pm-3pm].

Answer 1

编辑：

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如何让 Django 验证重叠的预订？

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可以为模型添加特殊方法validate_unique：

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    from django.db import models\n    from django.core.validators import ValidationError\n    from django.forms.forms import NON_FIELD_ERRORS\n\n    class Dish(models.Model):\n        name = models.CharField(\'Dish name\', max_length=200)\n\n    class Menu(models.Model):\n        user_id = models.IntegerField()\n        dish = models.ForeignKey(Dish)\n        order_start_time = models.DateTimeField()\n        order_end_time = models.DateTimeField()\n\n        def validate_unique(self, *args, **kwargs):\n            # call inherited unique validators\n            super().validate_unique(*args, **kwargs)  # or super(Menu, self) for Python2.7\n            # query if DB already has object with same dish \n            # and overlapping reservation \n            # [order_start_time, order_end_time]\n            qs = self.__class__._default_manager.filter(\n                    order_start_time__lte=self.order_end_time,\n                    order_end_time__gte=self.order_start_time,\n                    dish=self.dish,\n                )\n            # and this object is not the same we are working with\n            if not self._state.adding and self.pk is not None:\n                qs = qs.exclude(pk=self.pk)\n            if qs.exists():\n                raise ValidationError({\n                    NON_FIELD_ERRORS: [\'Overlapping order dates for dish\'],\n                    })\n

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让我们在控制台中尝试一下：

\n\n

    from core.models import *\n    m=Menu(user_id=1, dish_id=1, order_start_time=\'2016-03-22 10:00\', order_end_time=\'2016-03-22 15:00\')\n    m.validate_unique()\n    # no output here - all is ok\n    m.save()\n    print(m.id)\n    8\n\n    # lets add duplicate\n    m=Menu(user_id=1, dish_id=1, order_start_time=\'2016-03-22 12:00\', order_end_time=\'2016-03-22 13:00\')\n    m.validate_unique()\n    Traceback (most recent call last):\n       File "<console>", line 1, in <module>\n       File "/Users/el/tmp/hypothesis_test/menu/core/models.py", line 29, in validate_unique\n           NON_FIELD_ERRORS: [\'Overlapping order dates for dish\'],\n    django.core.exceptions.ValidationError: {\'__all__\': [\'Overlapping order dates for dish\']}\n\n    # excellent!  dup is found!\n\n    # But! Django helps you find dups but allows you to add them to db if you want it!\n    # It\'s responsibility of your application not to add duplicates.\n\n    m.save()\n    print(m.id)\n    9\n

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如何确保没有人可以添加重复项？

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在这种情况下，您需要在数据库级别进行约束。

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在 PostgreSQL 控制台中：

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    CREATE EXTENSION btree_gist;\n\n    -- our table:\n    SELECT * FROM core_menu;\n    id | user_id |    order_start_time    |     order_end_time     | dish_id\n   ----+---------+------------------------+------------------------+---------\n     8 |       1 | 2016-03-22 13:00:00+03 | 2016-03-22 18:00:00+03 |       1\n     9 |       1 | 2016-03-22 15:00:00+03 | 2016-03-22 16:00:00+03 |       1\n\n    DELETE FROM core_menu WHERE id=9; -- we should remove dups before adding unique constraint\n\n    ALTER TABLE core_menu \n        ADD CONSTRAINT core_menu_exclude_dish_same_tstzrange_constr \n            EXCLUDE USING gist (dish_id WITH =, tstzrange(order_start_time, order_end_time)  WITH &&);\n

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现在让我们创建重复的对象并将其添加到数据库中：

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    m=Menu(user_id=1, dish_id=1, order_start_time=\'2016-03-22 13:00\', order_end_time=\'2016-03-22 14:00\')\n\n    m.save()\n    Traceback (most recent call last):\n       File "/Users/el/tmp/hypothesis_test/venv/lib/python3.5/site-packages/django/db/backends/utils.py", line 64, in execute\n         return self.cursor.execute(sql, params)\n    psycopg2.IntegrityError: \xd0\x9e\xd0\xa8\xd0\x98\xd0\x91\xd0\x9a\xd0\x90:  \xd0\xba\xd0\xbe\xd0\xbd\xd1\x84\xd0\xbb\xd0\xb8\xd0\xba\xd1\x82\xd1\x83\xd1\x8e\xd1\x89\xd0\xb5\xd0\xb5 \xd0\xb7\xd0\xbd\xd0\xb0\xd1\x87\xd0\xb5\xd0\xbd\xd0\xb8\xd0\xb5 \xd0\xba\xd0\xbb\xd1\x8e\xd1\x87\xd0\xb0 \xd0\xbd\xd0\xb0\xd1\x80\xd1\x83\xd1\x88\xd0\xb0\xd0\xb5\xd1\x82 \xd0\xbe\xd0\xb3\xd1\x80\xd0\xb0\xd0\xbd\xd0\xb8\xd1\x87\xd0\xb5\xd0\xbd\xd0\xb8\xd0\xb5-\xd0\xb8\xd1\x81\xd0\xba\xd0\xbb\xd1\x8e\xd1\x87\xd0\xb5\xd0\xbd\xd0\xb8\xd0\xb5 "core_menu_exclude_dish_same_tstzrange_constr"\n    DETAIL:  Key (dish_id, tstzrange(order_start_time, order_end_time))=(1, ["2016-03-22 13:00:00+00","2016-03-22 14:00:00+00")) conflicts with existing key (dish_id, tstzrange(order_start_time, order_end_time))=(1, ["2016-03-22 10:00:00+00","2016-03-22 15:00:00+00")).\n

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太棒了！\n现在数据在程序和数据库级别进行了验证。

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