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django - 在 Django 中将两个不相关的表/模型与相同的主键组合起来

问题描述

我有两个具有相同主键的不相关表。

ip            mac
11.11.11.11   48-C0-09-1F-9B-54
33.33.33.33   4E-10-A3-BC-B8-9D
44.44.44.44   CD-00-60-08-56-2A
55.55.55.55   23-CE-D3-B1-39-A6

ip            type     owner
22.22.22.22   laptop   John Doe
33.33.33.33   server   XYZ Department
44.44.44.44   VM       Mary Smith
66.66.66.66   printer  ZWV Department

第一个表每分钟自动刷新一次。我无法更改数据库结构或填充它的脚本。

两个表都具有ipPRIMARY KEY。

在一个视图中，我想显示一个这样的表格：

ip           mac               type    owner          Alert
11.11.11.11  48-C0-09-1F-9B-54                        Unauthorized
55.55.55.55  23-CE-D3-B1-39-A6                        Unauthorized
22.22.22.22                    laptop  John Doe       Down
66.66.66.66                    printer ZWV Department Down
33.33.33.33  4E-10-A3-BC-B8-9D server  XYZ Department OK
44.44.44.44  CD-00-60-08-56-2A VM      Mary Smith     OK

我该如何建模？我应该将两个主键之一作为另一个主键的外键吗？

一旦代码运行起来，就会有很多数据，所以我想确保它足够快。

检索数据的最快方法是什么？

更新：

我尝试使用OneToOneField第二张桌子。

这有助于我获取两个表中的记录，以及未经授权的设备的记录（第二个表中缺少 IP）：

ip           mac               type    owner          Alert
11.11.11.11  48-C0-09-1F-9B-54                        Unauthorized
55.55.55.55  23-CE-D3-B1-39-A6                        Unauthorized
33.33.33.33  4E-10-A3-BC-B8-9D server  XYZ Department OK
44.44.44.44  CD-00-60-08-56-2A VM      Mary Smith     OK

但我无法获取已关闭的设备（第一个表中缺少 IP）：

22.22.22.22                    laptop  John Doe       Down
66.66.66.66                    printer ZWV Department Down

我在这里寻求帮助，但似乎无法完成OneToOneField

标签： djangodjango-modelslegacy-database

解决方案

大概的概念

您可以使用qs.union：

  • 创建 2 个模型，它们之间没有任何关系。不要忘记使用class Meta: managed = False
  • 从第一个模型中选择，用子查询注释并用第二个模型联合：
from django.db import models
from django.db.models import F, OuterRef, Subquery, Value
from django.db.models.functions import Coalesce

# OperationalDevice fields: ip, mac
# AllowedDevice fields: ip, type, owner

USE_EMPTY_STR_AS_DEFAULT = True

null_char_field = models.CharField(null=True)
if USE_EMPTY_STR_AS_DEFAULT:
    default_value = ''
else:
    default_value = None

# By default Expressions treat strings as "field_name" so if you want to use
# empty string as a second argument for Coalesce, then you should wrap it in
# `Value()`.
# `None` can be used there without wrapping in `Value()`, but in
# `.annotate(type=NoneValue)` it still should be wrapped, so it's easier to
# just "always wrap".
default_value = Value(default_value, output_field=null_char_field)

operational_devices_subquery = OperationalDevice.objects.filter(ip=OuterRef('ip'))


qs1 = (
    AllowedDevice.objects
    .all()
    .annotate(
        mac=Coalesce(
            Subquery(operational_devices_subquery.values('mac')[:1]),
            default_value,
            output_field=null_char_field,
        ),
    )
)

qs2 = (
    OperationalDevice.objects
    .exclude(
        ip__in=qs1.values('ip'),
    )
    .annotate(
        type=default_value,
        owner=default_value,
    )
)

final_qs = qs1.union(qs2)

多个领域的通用方法

更复杂但“通用”的方法可以使用Model._meta.get_fields(). 对于“第二个”模型具有超过 1 个额外字段（不仅ip,mac）的情况，它会更容易使用。示例代码（未经测试，但给人的印象一般）：

# One more import:
from django.db.models.fields import NOT_PROVIDED

common_field_name = 'ip'

# OperationalDevice fields: ip, mac, some_more_fields ...
# AllowedDevice fields: ip, type, owner

operational_device_fields = OperationalDevice._meta.get_fields()
operational_device_fields_names = {_f.name for _f in operational_device_fields}  # or set((_f.name for ...))

allowed_device_fields = AllowedDevice._meta.get_fields()
allowed_device_fields_names = {_f.name for _f in allowed_device_fields}  # or set((_f.name for ...))

operational_devices_subquery = OperationalDevice.objects.filter(ip=OuterRef(common_field_name))

left_joined_qs = (  # "Kind-of". Assuming AllowedDevice to be "left" and OperationalDevice to be "right"
    AllowedDevice.objects
    .all()
    .annotate(
        **{
            _f.name: Coalesce(
                Subquery(operational_devices_subquery.values(_f.name)[1]),
                Value(_f.get_default()),  # Use defaults from model definition
                output_field=_f,
            )
            for _f in operational_device_fields
            if _f.name not in allowed_device_fields_names
            # NOTE: if fields other than `ip` "overlap", then you might consider
            # changing logic here. Current implementation keeps fields from the
            # AllowedDevice
        }
        # Unpacked dict is partially equivalent to this:
        # mac=Coalesce(
        #     Subquery(operational_devices_subquery.values('mac')[:1]),
        #     default_for_mac_eg_fallback_text_value,
        #     output_field=null_char_field,
        # ),
        # other_field = Coalesce(...),
        # ...
    )
)

lonely_right_rows_qs = (
    OperationalDevice.objects
    .exclude(
        ip__in=AllowedDevice.objects.all().values(common_field_name),
    )
    .annotate(
        **{
            _f.name: Value(_f.get_default(), output_field=_f),  # Use defaults from model definition
            for _f in allowed_device_fields
            if _f.name not in operational_device_fields_names
            # NOTE: See previous NOTE
        }
    )
)

final_qs = left_joined_qs.union(lonely_right_rows_qs)

将 OneToOneField 用于“更好”的 SQL

理论上你可以使用device_info = models.OneToOneField(OperationalDevice, db_column='ip', primary_key=True, related_name='status_info'): in AllowedDevice。在这种情况下，您的第一个 QS 可以在不使用的情况下定义Subquery：

from django.db.models import F

# Now 'ip' is not in field names ('device_info' is there), so add it:
allowed_device_fields_names.add(common_field_name)

# NOTE: I think this approach will result in a more compact SQL query without 
# multiple `(SELECT "some_field" FROM device_info_table ... ) as "some-field"`.
# This also might result in better query performance.
honest_join_qs = (
    AllowedDevice.objects
    .all()
    .annotate(
        **{
            _f.name: F(f'device_info__{_f.name}')
            for _f in operational_device_fields
            if _f.name not in allowed_device_fields_names
        }
    )
)

final_qs = honest_join_qs.union(lonely_right_rows_qs)
# or:
# final_qs = honest_join_qs.union(
#     OperationalDevice.objects.filter(status_info__isnull=True).annotate(**missing_fields_annotation)
# )
# I'm not sure which approach is better performance-wise...
# Commented one will use something like:
# `SELECT ... FROM "device_info_table" LEFT OUTER JOIN "status_info_table" ON ("device_info_table"."ip" = "status_info_table"."ip") WHERE "status_info_table"."ip" IS NULL
#
# So it might be a little better than first with `union(QS.exclude(ip__in=honest_join_qs.values('ip'))`.
# Because later uses SQL like this:
# `SELECT ... FROM "device_info_table" WHERE NOT ip IN (SELECT ip FROM "status_info_table")`
#
# But it's better to measure timings of both approaches to be sure.
# @GrannyAching, can you compare them and tell in the comments which one is better ?

PS要自动化模型定义，您可以使用manage.py inspectdb

PPS 也许使用自定义的多表继承OneToOneField(..., parent_link=True)可能比使用union.

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