我知道Django不支持跨多个数据库的外键（最初是Django 1.3文档）
但我正在寻找解决方法。
什么不起作用
我在单独的数据库中有两个模型。
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中的模型1：
from django.db import models
class Fruit(models.Model):
name = models.CharField(max_length=20)
news_app / models.py中的模型2：
from django.db import models
class Article(models.Model):
fruit = models.ForeignKey('fruit_app.Fruit')
intro = models.TextField()
尝试在管理员中添加“文章”会出现以下错误，因为它正在Fruit错误的数据库（'news_db'）上寻找模型：
DatabaseError at /admin/news_app/article/add/
(1146, "Table 'fkad_news.fruit_app_fruit' doesn't exist")
方法1：子类IntegerField
我创建了一个自定义字段ForeignKeyAcrossDb，它是IntegerField的子类。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)
我将Article模型更改为：
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，有时是Fruit对象。我希望它始终是一个Fruit对象。我需要做些什么才能使访问Article.fruit始终返回Fruit对象？
作为解决方法，我添加了一个fruit_obj属性，但如果可能的话，我想消除它：
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
第二次尝试，我尝试对ForeignKey字段进行子类化。我修改ReverseSingleRelatedObjectDescriptor为使用forced_using的模型管理器上指定的数据库Fruit。我还删除validate()了ForeignKey子类上的方法。这种方法并没有同样的问题的方法1.代码在GitHub上的：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添加路由器
此解决方案使用了一个额外的路由器fruit_app。此解决方案不需要对ForeignKey方法2中进行的修改。在中查看了Django的默认路由行为后django.db.utils.ConnectionRouter，我们发现，即使我们期望默认情况下fruit_app位于'default'数据库中，instance传递给db_for_read外键查找的提示也会将其放在该'news_db'数据库。我们添加了第二个路由器，以确保fruit_app始终从'default'数据库中读取模型。甲ForeignKey亚类仅用于“修复”的ForeignKey.validate()方法。（如果Django想跨数据库支持外键，我会说这是Django错误。）代码位于github上，网址为：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
- 包含大量信息的django-users列表上的线程：http : //groups.google.com/group/django-users/browse_thread/thread/74bcd1afdeb2f0/0fdfce061124b915
- 多数据库文档的修订历史记录：http : //code.djangoproject.com/log/django/trunk/docs/topics/db/multi-db.txt?verbose=on
更新资料
在对路由器进行了一些调整之后，我们实现了最后一种方法。整个实现过程非常痛苦，这使我们认为我们必须做错了。在TODO列表中正在为此编写单元测试。