django multi db routing不适用于多个模式

Question

我有django在oracle后端运行.我需要使用两个模式 - 一个用于遗留DB,第二个用于所有django相关表.

所以这是我的设置.数据显示:

APPS_DB = 'apps'
DATABASES = {

    'default' : { 
        'ENGINE': 'django.db.backends.oracle'
        'NAME': 'django',                      
        'USER': 'django-tables',                      
        'PASSWORD': '****',                  
        'HOST': 'localhost',                       
        'PORT': '1531',                     
    },

    APPS_DB : { 
        'ENGINE': 'django.db.backends.oracle', 
        'NAME': 'django',                      
        'USER': 'legacy-stuff',                      
        'PASSWORD': '****',                 
        'HOST': 'localhost',                     
        'PORT': '1531',                      
     },
}

我还定义了路由器:

class MyRouter(object):
    """A router to control all database operations on models"""

def __init__(self):
    aux = []
    for app in settings.INSTALLED_APPS:
        if not app.endswith('myapp'):
            aux.append(app)
    self.djangoStuff = tuple(map(lambda x: x[x.rfind('.')+1:], aux))

def is_django_stuff(self, model):
    return model._meta.app_label in self.djangoStuff

def db_for_read(self, model, **hints):
    "Point all django apps models to separate DB"
    logger.info("READ from " + model._meta.app_label)
    if self.is_django_stuff(model):
        logger.info("Will be directed to default DB")
        return None
    logger.info("Will be directed to legacy DB")    
    return settings.APPS_DB

def db_for_write(self, model, **hints):
    "Point all django apps models to separate DB"
    logger.info("WRITE")
    if self.is_django_stuff(model):
        return None
    return settings.APPS_DB

def allow_relation(self, obj1, obj2, **hints):
    "Allow any relation"
    logger.info("ALLOW REL")
    return True

def allow_syncdb(self, db, model):
    "Allow syncdb for all managed objects"
    logger.info("ALLOW SYNC")
    if db == 'default' and self.is_django_stuff(model):
        return True
    if db != 'default' and not self.is_django_stuff(model):
        return True
    return False

现在我有一个非常简单的模型:

class Poll(models.Model):
    question = models.CharField(max_length=200)
    user = models.ForeignKey(User)
    pub_date = models.DateTimeField('date published')

我做了两个syncdbs:

python manage.py syncdb
python manage.py syndb --database apps

一切都很顺利.然后我使用'python manage.py shell'创建poll对象

superuser = User.objects.all()[0]
p = Poll(question="foo", user = superuser, pub_date = datetime.now())
p.save()

我尝试从民意调查中检索用户:

  a = Poll.objects.all()
  b = len(a)
  b = a[0]
  c = b.artist

我在路由器中启用了日志记录,因此我看到最后一个查询将被定向到正确的数据库:

READ from myapp
Will be directed to apps DB
READ from myapp
Will be directed to apps DB
READ from auth
Will be directed to default DB

我甚至可以看到实际的SQL语句:

(0.005) SELECT "AUTH_USER"."ID", "AUTH_USER"."USERNAME", "AUTH_USER"."FIRST_NAME",    "AUTH_USER"."LAST_NAME", "AUTH_USER"."EMAIL", "AUTH_USER"."PASSWORD", "AUTH_USER"."IS_STAFF", "AUTH_USER"."IS_ACTIVE", "AUTH_USER"."IS_SUPERUSER", "AUTH_USER"."LAST_LOGIN", "AUTH_USER"."DATE_JOINED" FROM "AUTH_USER" WHERE "AUTH_USER"."ID" = :arg0 ; args=(1,)

但是我收到了错误:

  File "<console>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/Django-1.4.1-py2.7.egg/django/db/models/fields/related.py", line 350, in __get__
    rel_obj = qs.get(**params)
  File "/usr/local/lib/python2.7/dist-packages/Django-1.4.1-py2.7.egg/django/db/models/query.py", line 361, in get
    num = len(clone)
  File "/usr/local/lib/python2.7/dist-packages/Django-1.4.1-py2.7.egg/django/db/models/query.py", line 85, in __len__
    self._result_cache = list(self.iterator())
  File "/usr/local/lib/python2.7/dist-packages/Django-1.4.1-py2.7.egg/django/db/models/query.py", line 291, in iterator
    for row in compiler.results_iter():
  File "/usr/local/lib/python2.7/dist-packages/Django-1.4.1-py2.7.egg/django/db/models/sql/compiler.py", line 763, in results_iter
    for rows in self.execute_sql(MULTI):
  File "/usr/local/lib/python2.7/dist-packages/Django-1.4.1-py2.7.egg/django/db/models/sql/compiler.py", line 818, in execute_sql
    cursor.execute(sql, params)
  File "/usr/local/lib/python2.7/dist-packages/Django-1.4.1-py2.7.egg/django/db/backends/util.py", line 40, in execute
return self.cursor.execute(sql, params)
  File "/usr/local/lib/python2.7/dist-packages/Django-1.4.1-py2.7.egg/django/db/backends/oracle/base.py", line 675, in execute
    return self.cursor.execute(query, self._param_generator(params))
DatabaseError: ORA-00942: table or view does not exist

所以我的问题是 - 我做错了什么？

Answer 1

跨数据库外键本质上是无效的，因为 Django 正在建模一个具有引用完整性的“正确”关系数据库，并且如果模型存储在完全不同的物理存储中，则无法在数据库级别强制执行。

无论如何，出于这个原因，Django 必须假设任何对象都存在于与您最初检索的对象相同的数据库中。就您而言，它从旧数据库中获取了您的 Poll 对象，因此它也必须在那里查找您的用户（或艺术家或其他任何东西）。

对于像这样的简单查询，很容易解决，例如：

poll = Poll.objects.all()[0]
user_id = poll.user_id # _id after the name of your "Foreign Key" field - which cannot really be an FK
user = User.objects.get(user_id) # This will be a new query and can use a different database, it will check the router

对于更复杂的查询（连接等），您经常会发现需要构建列表或 id 集，并使用 filter(id__in=your_list_of_ids) 进行查询。

根据记录的数量，执行此操作可能会导致性能或内存使用量下降。（但在某些情况下，您的查询实际上会比原始连接快得多，这完全取决于您的应用程序。）您可能需要将 id 列表分成批次，否则您的查询可能会变得太长，等等。但没有这些问题都是无法克服的。

当您处理来自另一个数据库的 id 时，您需要强制执行引用完整性。有时您需要设置批处理来处理整理数据。

这一切听起来都是错误的，但这种关注点分离，特别是如果您可以限制依赖关系，并且只在一个方向上，则可能是完全正确的方法。