如何解决在Django中跨数据库缺less外键支持的问题

我知道Django不支持跨多个数据库的外键 （原来是Django 1.3文档）

但是我正在寻找解决方法。

什么都行不通

我有两个模型，每个在一个单独的数据库。

routers.py：

class NewsRouter(object): def db_for_read(self, model, **hints): if model._meta.app_label == 'news_app': return 'news_db' return None def db_for_write(self, model, **hints): if model._meta.app_label == 'news_app': return 'news_db' return None def allow_relation(self, obj1, obj2, **hints): if obj1._meta.app_label == 'news_app' or obj2._meta.app_label == 'news_app': return True return None def allow_syncdb(self, db, model): if db == 'news_db': return model._meta.app_label == 'news_app' elif model._meta.app_label == 'news_app': return False return None 

fruit_app / models.py中的Model 1：

 from django.db import models class Fruit(models.Model): name = models.CharField(max_length=20) 

news_app / models.py中的Model 2：

 from django.db import models class Article(models.Model): fruit = models.ForeignKey('fruit_app.Fruit') intro = models.TextField() 

尝试在pipe理员中添加“文章”会导致以下错误，因为它在错误的数据库（ 'news_db' ）上查找Fruit模型：

 DatabaseError at /admin/news_app/article/add/ (1146, "Table 'fkad_news.fruit_app_fruit' doesn't exist") 

方法1：子类IntegerField

我创build了一个自定义字段ForeignKeyAcrossDb，它是IntegerField的一个子类。 代码在github上： https : //github.com/saltycrane/django-foreign-key-across-db-testproject/tree/integerfield_subclass

fields.py：

 from django.db import models class ForeignKeyAcrossDb(models.IntegerField): ''' Exists because foreign keys do not work across databases ''' def __init__(self, model_on_other_db, **kwargs): self.model_on_other_db = model_on_other_db super(ForeignKeyAcrossDb, self).__init__(**kwargs) def to_python(self, value): # TODO: this db lookup is duplicated in get_prep_lookup() if isinstance(value, self.model_on_other_db): return value else: return self.model_on_other_db._default_manager.get(pk=value) def get_prep_value(self, value): if isinstance(value, self.model_on_other_db): value = value.pk return super(ForeignKeyAcrossDb, self).get_prep_value(value) def get_prep_lookup(self, lookup_type, value): # TODO: this db lookup is duplicated in to_python() if not isinstance(value, self.model_on_other_db): value = self.model_on_other_db._default_manager.get(pk=value) return super(ForeignKeyAcrossDb, self).get_prep_lookup(lookup_type, value) 

我把我的文章模型改为：

 class Article(models.Model): fruit = ForeignKeyAcrossDb(Fruit) intro = models.TextField() 

问题是，有时当我访问Article.fruit时，它是一个整数，有时它是水果对象。 我希望它永远是一个水果对象。 我需要做些什么来使访问Article.fruit总是返回一个水果对象？

作为我的解决方法的解决方法，我添加了一个fruit_obj属性，但是如果可能，我想消除这个：

 class Article(models.Model): fruit = ForeignKeyAcrossDb(Fruit) intro = models.TextField() # TODO: shouldn't need fruit_obj if ForeignKeyAcrossDb field worked properly @property def fruit_obj(self): if not hasattr(self, '_fruit_obj'): # TODO: why is it sometimes an int and sometimes a Fruit object? if isinstance(self.fruit, int) or isinstance(self.fruit, long): print 'self.fruit IS a number' self._fruit_obj = Fruit.objects.get(pk=self.fruit) else: print 'self.fruit IS NOT a number' self._fruit_obj = self.fruit return self._fruit_obj def fruit_name(self): return self.fruit_obj.name 

方法2：子类ForeignKey字段

作为第二次尝试，我尝试了子类的ForeignKey字段。 我修改了ReverseSingleRelatedObjectDescriptor以使用在Fruit的模型pipe理器上由forced_using指定的数据库。 我也删除了ForeignKey子类的validate()方法。 这种方法没有问题的方法1. github上的代码： https ： //github.com/saltycrane/django-foreign-key-across-db-testproject/tree/foreignkey_subclass

fields.py：

 from django.db import models from django.db import router from django.db.models.query import QuerySet class ReverseSingleRelatedObjectDescriptor(object): # This class provides the functionality that makes the related-object # managers available as attributes on a model class, for fields that have # a single "remote" value, on the class that defines the related field. # In the example "choice.poll", the poll attribute is a # ReverseSingleRelatedObjectDescriptor instance. def __init__(self, field_with_rel): self.field = field_with_rel def __get__(self, instance, instance_type=None): if instance is None: return self cache_name = self.field.get_cache_name() try: return getattr(instance, cache_name) except AttributeError: val = getattr(instance, self.field.attname) if val is None: # If NULL is an allowed value, return it. if self.field.null: return None raise self.field.rel.to.DoesNotExist other_field = self.field.rel.get_related_field() if other_field.rel: params = {'%s__pk' % self.field.rel.field_name: val} else: params = {'%s__exact' % self.field.rel.field_name: val} # If the related manager indicates that it should be used for # related fields, respect that. rel_mgr = self.field.rel.to._default_manager db = router.db_for_read(self.field.rel.to, instance=instance) if getattr(rel_mgr, 'forced_using', False): db = rel_mgr.forced_using rel_obj = rel_mgr.using(db).get(**params) elif getattr(rel_mgr, 'use_for_related_fields', False): rel_obj = rel_mgr.using(db).get(**params) else: rel_obj = QuerySet(self.field.rel.to).using(db).get(**params) setattr(instance, cache_name, rel_obj) return rel_obj def __set__(self, instance, value): raise NotImplementedError() class ForeignKeyAcrossDb(models.ForeignKey): def contribute_to_class(self, cls, name): models.ForeignKey.contribute_to_class(self, cls, name) setattr(cls, self.name, ReverseSingleRelatedObjectDescriptor(self)) if isinstance(self.rel.to, basestring): target = self.rel.to else: target = self.rel.to._meta.db_table cls._meta.duplicate_targets[self.column] = (target, "o2m") def validate(self, value, model_instance): pass 

fruit_app / models.py：

 from django.db import models class FruitManager(models.Manager): forced_using = 'default' class Fruit(models.Model): name = models.CharField(max_length=20) objects = FruitManager() 

news_app / models.py：

 from django.db import models from foreign_key_across_db_testproject.fields import ForeignKeyAcrossDb from foreign_key_across_db_testproject.fruit_app.models import Fruit class Article(models.Model): fruit = ForeignKeyAcrossDb(Fruit) intro = models.TextField() def fruit_name(self): return self.fruit.name 

方法2a：为fruit_app添加一个路由器

此解决scheme使用额外的路由器fruit_app 。 这个解决scheme不需要对方法2中所需的ForeignKey进行修改。在django.db.utils.ConnectionRouter查看Django的默认路由行为之后，我们发现即使我们预期fruit_app 'default'在'default'数据库上，传递给db_for_read的外键查找的instance提示把它放在'news_db'数据库上。 我们添加了第二个路由器，以确保fruit_app模型总是从'default'数据库中读取。 ForeignKey子类仅用于“修复” ForeignKey.validate()方法。 （如果Django想要跨数据库支持外键，我会说这是一个Django错误。）代码在github上： https : //github.com/saltycrane/django-foreign-key-across-db-testproject

routers.py：

 class NewsRouter(object): def db_for_read(self, model, **hints): if model._meta.app_label == 'news_app': return 'news_db' return None def db_for_write(self, model, **hints): if model._meta.app_label == 'news_app': return 'news_db' return None def allow_relation(self, obj1, obj2, **hints): if obj1._meta.app_label == 'news_app' or obj2._meta.app_label == 'news_app': return True return None def allow_syncdb(self, db, model): if db == 'news_db': return model._meta.app_label == 'news_app' elif model._meta.app_label == 'news_app': return False return None class FruitRouter(object): def db_for_read(self, model, **hints): if model._meta.app_label == 'fruit_app': return 'default' return None def db_for_write(self, model, **hints): if model._meta.app_label == 'fruit_app': return 'default' return None def allow_relation(self, obj1, obj2, **hints): if obj1._meta.app_label == 'fruit_app' or obj2._meta.app_label == 'fruit_app': return True return None def allow_syncdb(self, db, model): if db == 'default': return model._meta.app_label == 'fruit_app' elif model._meta.app_label == 'fruit_app': return False return None 

fruit_app / models.py：

 from django.db import models class Fruit(models.Model): name = models.CharField(max_length=20) 

news_app / models.py：

 from django.db import models from foreign_key_across_db_testproject.fields import ForeignKeyAcrossDb from foreign_key_across_db_testproject.fruit_app.models import Fruit class Article(models.Model): fruit = ForeignKeyAcrossDb(Fruit) intro = models.TextField() def fruit_name(self): return self.fruit.name 

fields.py：

 from django.core import exceptions from django.db import models from django.db import router class ForeignKeyAcrossDb(models.ForeignKey): def validate(self, value, model_instance): if self.rel.parent_link: return models.Field.validate(self, value, model_instance) if value is None: return using = router.db_for_read(self.rel.to, instance=model_instance) # is this more correct than Django's 1.2.5 version? qs = self.rel.to._default_manager.using(using).filter( **{self.rel.field_name: value} ) qs = qs.complex_filter(self.rel.limit_choices_to) if not qs.exists(): raise exceptions.ValidationError(self.error_messages['invalid'] % { 'model': self.rel.to._meta.verbose_name, 'pk': value}) 

附加信息

• 在包含大量信息的django用户列表中进行讨论： http : //groups.google.com/group/django-users/browse_thread/thread/74bcd1afdeb2f0/0fdfce061124b915
• 多版本文档的修订历史logging： http : //code.djangoproject.com/log/django/trunk/docs/topics/db/multi-db.txt?verbose=on

更新

我们在调整我们的路由器之后再执行最后一个方法。 整个实施是非常痛苦的，这使我们认为我们必须做错了。 在TODO列表上正在为此编写unit testing。

Answer 1

我知道，Djano-nosql支持密钥等，虽然从http://www.allbuttonspressed.com/projects/django-dbindexer魔术。; 也许有些可以帮助。

从描述：

“你可以告诉dbindexer哪些模型和字段应该支持这些查询，并且会为你维护所需的索引。”

-Kerry

Answer 2

至于ForeignKeyAcrossDb部分，你不可能在__init__里面调整你的类吗？ 检查相应的字段是否为Integer如果不是，则从数据库加载它，或执行任何其他所需的操作。 Python __class__ es可以在运行时改变，没有太大的问题。

Answer 3

有一天，我的头脑突然崩溃，我设法让我的外键在同一个银行！

可以对FORM进行更改，以在另一家银行寻找一个FOREIGN KEY！

首先，添加FIHDS RECHARGE，直接（破解）我的forms，在函数_ init _

app.form.py

 # -*- coding: utf-8 -*- from django import forms import datetime from app_ti_helpdesk import models as mdp #classe para formulario de Novo HelpDesk class FormNewHelpDesk(forms.ModelForm): class Meta: model = mdp.TblHelpDesk fields = ( "problema_alegado", "cod_direcionacao", "data_prevista", "hora_prevista", "atendimento_relacionado_a", "status", "cod_usuario", ) def __init__(self, *args, **kwargs): #------------------------------------- # using remove of kwargs #------------------------------------- db = kwargs.pop("using", None) # CASE use Unique Keys self.Meta.model.db = db super(FormNewHelpDesk, self).__init__(*args,**kwargs) #------------------------------------- # recreates the fields manually from copy import deepcopy self.fields = deepcopy( forms.fields_for_model( self.Meta.model, self.Meta.fields, using=db ) ) # #------------------------------------- #### follows the standard template customization, if necessary self.fields['problema_alegado'].widget.attrs['rows'] = 3 self.fields['problema_alegado'].widget.attrs['cols'] = 22 self.fields['problema_alegado'].required = True self.fields['problema_alegado'].error_messages={'required': 'Necessário informar o motivo da solicitação de ajuda!'} self.fields['data_prevista'].widget.attrs['class'] = 'calendario' self.fields['data_prevista'].initial = (datetime.timedelta(4)+datetime.datetime.now().date()).strftime("%Y-%m-%d") self.fields['hora_prevista'].widget.attrs['class'] = 'hora' self.fields['hora_prevista'].initial =datetime.datetime.now().time().strftime("%H:%M") self.fields['status'].initial = '0' #aberto self.fields['status'].widget.attrs['disabled'] = True self.fields['atendimento_relacionado_a'].initial = '07' self.fields['cod_direcionacao'].required = True self.fields['cod_direcionacao'].label = "Direcionado a" self.fields['cod_direcionacao'].initial = '2' self.fields['cod_direcionacao'].error_messages={'required': 'Necessário informar para quem é direcionado a ajuda!'} self.fields['cod_usuario'].widget = forms.HiddenInput() 

从视图调用窗体

app.view.py

 form = forms.FormNewHelpDesk(request.POST or None, using=banco) 

现在，在源代码DJANGO的变化

只有ForeignKey，ManyToManyField和OneToOneFieldtypes的字段可以使用'using'，所以添加了一个IF …

django.forms.models.py

 # line - 133: add using=None def fields_for_model(model, fields=None, exclude=None, widgets=None, formfield_callback=None, using=None): # line - 159 if formfield_callback is None: #---------------------------------------------------- from django.db.models.fields.related import (ForeignKey, ManyToManyField, OneToOneField) if type(f) in (ForeignKey, ManyToManyField, OneToOneField): kwargs['using'] = using formfield = f.formfield(**kwargs) #---------------------------------------------------- elif not callable(formfield_callback): raise TypeError('formfield_callback must be a function or callable') else: formfield = formfield_callback(f, **kwargs) 

改变跟踪文件

django.db.models.base.py

改变

 # line 717 qs = model_class._default_manager.filter(**lookup_kwargs) 

对于

 # line 717 qs = model_class._default_manager.using(getattr(self, 'db', None)).filter(**lookup_kwargs) 

准备好了：D

Answer 4

您可以在其中包含跨数据库查询的数据库中查看视图，然后在单独的文件中定义视图的模型以保持syncdb正常工作。

开心编程。 🙂

Answer 5

外键字段意味着你可以 – 通过join查询关系，即水果_name – 检查参照完整性 – 确保删除后的参照完整性 – pipe理员原始ID查找function – （更多…）

第一个用例总是有问题的。 在代码库中可能还有一些其他的外键特例也是行不通的。

我运行一个相当大的django网站，我们目前正在使用一个普通的整数字段。 现在，我认为子类化整数字段并将ID添加到对象转换将是最简单的（在1.2中需要修补一些django，希望现在有所改进），让我们知道我们find了什么解决scheme。

Answer 6

存在需要跨多个（5）数据库引用（大部分）静态数据的类似问题。 对ReversedSingleRelatedObjectDescriptor进行了一些更新，以允许设置相关的模型。 它没有实现反向关系atm。

 class ReverseSingleRelatedObjectDescriptor(object): """ This class provides the functionality that makes the related-object managers available as attributes on a model class, for fields that have a single "remote" value, on the class that defines the related field. Used with LinkedField. """ def __init__(self, field_with_rel): self.field = field_with_rel self.cache_name = self.field.get_cache_name() def __get__(self, instance, instance_type=None): if instance is None: return self try: return getattr(instance, self.cache_name) except AttributeError: val = getattr(instance, self.field.attname) if val is None: # If NULL is an allowed value, return it if self.field.null: return None raise self.field.rel.to.DoesNotExist other_field = self.field.rel.get_related_field() if other_field.rel: params = {'%s__pk' % self.field.rel.field_name: val} else: params = {'%s__exact' % self.field.rel.field_name: val} # If the related manager indicates that it should be used for related fields, respect that. rel_mgr = self.field.rel.to._default_manager db = router.db_for_read(self.field.rel.to, instance=instance) if getattr(rel_mgr, 'forced_using', False): db = rel_mgr.forced_using rel_obj = rel_mgr.using(db).get(**params) elif getattr(rel_mgr, 'use_for_related_fields', False): rel_obj = rel_mgr.using(db).get(**params) else: rel_obj = QuerySet(self.field.rel.to).using(db).get(**params) setattr(instance, self.cache_name, rel_obj) return rel_obj def __set__(self, instance, value): if instance is None: raise AttributeError("%s must be accessed via instance" % self.field.name) # If null=True, we can assign null here, but otherwise the value needs to be an instance of the related class. if value is None and self.field.null is False: raise ValueError('Cannot assign None: "%s.%s" does not allow null values.' % (instance._meta.object_name, self.field.names)) elif value is not None and not isinstance(value, self.field.rel.to): raise ValueError('Cannot assign "%r": "%s.%s" must be a "%s" instance.' % (value, instance._meta.object_name, self.field.name, self.field.rel.to._meta.object_name)) elif value is not None: # Only check the instance state db, LinkedField implies that the value is on a different database if instance._state.db is None: instance._state.db = router.db_for_write(instance.__class__, instance=value) # Is not used by OneToOneField, no extra measures to take here # Set the value of the related field try: val = getattr(value, self.field.rel.get_related_field().attname) except AttributeError: val = None setattr(instance, self.field.attname, val) # Since we already know what the related object is, seed the related object caches now, too. This avoids another # db hit if you get the object you just set setattr(instance, self.cache_name, value) if value is not None and not self.field.rel.multiple: setattr(value, self.field.related.get_cache_name(), instance) 

和

 class LinkedField(models.ForeignKey): """ Field class used to link models across databases. Does not ensure referrential integraty like ForeignKey """ def _description(self): return "Linked Field (type determined by related field)" def contribute_to_class(self, cls, name): models.ForeignKey.contribute_to_class(self, cls, name) setattr(cls, self.name, ReverseSingleRelatedObjectDescriptor(self)) if isinstance(self.rel.to, basestring): target = self.rel.to else: target = self.rel.to._meta.db_table cls._meta.duplicate_targets[self.column] = (target, "o2m") def validate(self, value, model_instance): pass 

Answer 7

灵感来自@Frans的评论。 我的解决方法是在业务层中执行此操作。 在给出这个问题的例子。 我将设置一个IntegerField上的Article ，作为“不做数据层的完整性检查”。

 class Fruit(models.Model): name = models.CharField() class Article(models.Model): fruit = models.IntegerField() intro = models.TextField() 

然后在应用程序代码（业务层）中尊重引用关系。 以Djangopipe理员为例，为了在文章的添加页面中显示水果作为select，您可以手动填充水果的选项列表。

 # admin.py in App article class ArticleAdmin(admin.ModelAdmin): class ArticleForm(forms.ModelForm): fields = ['fruit', 'intro'] # populate choices for fruit choices = [(obj.id, obj.name) for obj in Fruit.objects.all()] widgets = { 'fruit': forms.Select(choices=choices)} form = ArticleForm list_diaplay = ['fruit', 'intro'] 

当然，您可能需要照顾表单validation（完整性检查）。

Answer 8

我有一个新的django v1.10的解决scheme。 有两个部分。 它适用于django.admin和django.rest-framework。

1. inheritanceForeignKey类并创buildForeignKeyAcrossDb ，并根据此故障单和本文重写validate()函数。

 class ForeignKeyAcrossDb(models.ForeignKey): def validate(self, value, model_instance): if self.remote_field.parent_link: return super(models.ForeignKey, self).validate(value, model_instance) if value is None: return using = router.db_for_read(self.remote_field.model, instance=model_instance) qs = self.remote_field.model._default_manager.using(using).filter( **{self.remote_field.field_name: value} ) qs = qs.complex_filter(self.get_limit_choices_to()) if not qs.exists(): raise exceptions.ValidationError( self.error_messages['invalid'], code='invalid', params={ 'model': self.remote_field.model._meta.verbose_name, 'pk': value, 'field': self.remote_field.field_name, 'value': value, }, # 'pk' is included for backwards compatibility ) 

2. 在字段声明中，使用db_constraint=False ，例如，

album=ForeignKeyAcrossDb(Singer, db_constraint=False, on_delete=models.DO_NOTHING)

Answer 9

此解决scheme最初是为一个托pipe数据库编写的，其中迁移数据为一个或多个旧数据库，并且在数据库级别将Meta managed=False连接到相同的数据库。 如果一个db_table选项包含一个数据库名称加上由“` （MySQL） '或''' （其他数据库）正确引用的表格名称，例如db_table = '"DB2"."table_b"' ，那么它不再被引用通过Django查询由Django ORM正确编译，即使使用JOIN：

 class TableB(models.Model): .... class Meta: db_table = '`DB2`.`table_b`' # for MySQL # db_table = '"DB2"."table_b"' # for all other backends managed = False 

查询集：

 >>> qs = TableB.objects.all() >>> str(qs.query) 'SELECT "DB2"."table_b"."id" FROM DB2"."table_b"' 

Django的所有数据库后端都支持这个function。

（似乎我开始了一个重复的新问题 ，我的答案仍在继续。）