我一直试图在后端使用SQLAlchemy来获得一个像样的大小项目。我有多个文件的表模型，自己文件中的声明性基础，以及用于包装常见SQLAlchemy函数和驱动程序文件的帮助文件。
我上传了数据，然后决定添加一个列。由于这只是测试数据，我认为最简单的方法是删除所有表并开始新的...然后当我尝试重新创建模式和表时，公共声明性基类突然有空元数据。我通过导入类声明文件解决了这个问题 - 奇怪的是，因为我之前不需要那些导入 - 并且它能够成功地重新创建模式。
但是现在当我再次尝试创建对象时，我收到一个错误：
AttributeError: mapper
现在我完全糊涂了！有人能解释一下这里发生了什么吗？在我放弃架构之前它工作正常，现在我无法正常工作。
这是我设置的骨架：
base.py
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
models1.py
from base import Base
class Business(Base):
__tablename__ = 'business'
id = Column(Integer, primary_key=True)
models2.py：
from base import Base
class Category(Base):
__tablename__ = 'category'
id = Column(Integer, primary_key=True)
helper.py：
from base import Base
# I didn't need these two imports the first time I made the schema
# I added them after I was just getting an empty schema from base.Base
# but have no idea why they're needed now?
import models1
import models2
def setupDB():
engine = getDBEngine(echo=True) # also a wrapped func (omitted for space)
#instantiate the schema
try:
Base.metadata.create_all(engine, checkfirst=True)
logger.info("Successfully instantiated Database with model schema")
except:
logger.error("Failed to instantieate Database with model schema")
traceback.print_exc()
def dropAllTables():
engine = getDBEngine(echo=True)
# drop the schema
try:
Base.metadata.reflect(engine, extend_existing=True)
Base.metadata.drop_all(engine)
logger.info("Successfully dropped all the database tables in the schema")
except:
logger.error("Failed to drop all tables")
traceback.print_exc()
driver.py：
import models1
import models2
# ^ some code to get to this point
categories []
categories.append(
models2.Category(alias=category['alias'],
title=category['title']) # error occurs here
)
堆栈跟踪:(完整性）
File "./main.py", line 16, in <module>
yelp.updateDBFromYelpFeed(fname)
File "/Users/thomaseffland/Development/projects/health/pyhealth/pyhealth/data/sources/yelp.py", line 188, in updateDBFromYelpFeed
title=category['title'])
File "<string>", line 2, in __init__
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py", line 347, in _new_state_if_none
state = self._state_constructor(instance, self)
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 747, in __get__
obj.__dict__[self.__name__] = result = self.fget(obj)
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py", line 177, in _state_constructor
self.dispatch.first_init(self, self.class_)
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/event/attr.py", line 256, in __call__
fn(*args, **kw)
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 2825, in _event_on_first_init
configure_mappers()
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 2721, in configure_mappers
mapper._post_configure_properties()
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 1710, in _post_configure_properties
prop.init()
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/orm/interfaces.py", line 183, in init
self.do_init()
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1616, in do_init
self._process_dependent_arguments()
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/orm/relationships.py", line 1673, in _process_dependent_arguments
self.target = self.mapper.mapped_table
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 833, in __getattr__
return self._fallback_getattr(key)
File "/Users/thomaseffland/.virtualenvs/health/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 811, in _fallback_getattr
raise AttributeError(key)
AttributeError: mapper
我知道这篇文章很长，但我想提供完整的图片。首先，我很困惑为什么base.Base架构首先是空的。现在我很困惑为什么Categories对象缺少mapper！
非常感谢任何帮助/见解/建议，谢谢！
修改
因此模型文件和helper.py在supackage中，driver.py实际上是兄弟子包中的文件，其代码包含在一个函数中。此驱动程序函数由包级主文件调用。所以我不认为这可能是因为SQLAlchemy还没有时间初始化？ （如果我理解答案正确的话）这里是主文件的（相关部分）的样子：
main.py：
import models.helper as helper
helper.setupDB(echo=true) # SQLAlchemy echos the correct statements
import driverpackage.driver as driver
driver.updateDBFromFile(fname) # error occurs in here
和 driver.py 实际上如下：
import ..models.models1
import ..models.models2
def updateDBFromFile(fname):
# ^ some code to get to this point
categories []
categories.append(
models2.Category(alias=category['alias'],
title=category['title']) # error occurs here
)
# a bunch more code
编辑2：
我开始怀疑潜在的问题与我突然需要导入所有模型以在helper.py中设置架构的原因相同。如果我打印导入的模型对象的表，它们没有绑定的MetaData或模式：
print YelpCategory.__dict__['__table__'].__dict__
####
{'schema': None, '_columns': <sqlalchemy.sql.base.ColumnCollection object at 0x102312ef0>,
'name': 'yelp_category', 'description': 'yelp_category',
'dispatch': <sqlalchemy.event.base.DDLEventsDispatch object at 0x10230caf0>,
'indexes': set([]), 'foreign_keys': set([]),
'columns': <sqlalchemy.sql.base.ImmutableColumnCollection object at 0x10230fc58>,
'_prefixes': [],
'_extra_dependencies': set([]),
'fullname': 'yelp_category', 'metadata': MetaData(bind=None),
'implicit_returning': True,
'constraints': set([PrimaryKeyConstraint(Column('id', Integer(), table=<yelp_category>, primary_key=True, nullable=False))]), 'primary_key': PrimaryKeyConstraint(Column('id', Integer(), table=<yelp_category>, primary_key=True, nullable=False))}
我想知道为什么来自创建数据库的基础的元数据没有被绑定？
答案 0 :(得分：0)
我猜错误的发生是因为您在Python模块级执行代码。当Python导入模块时执行此代码。
将代码移至函数。
在SQLAlchemy正确初始化后调用该函数。
DBSession.configure(bind=engine)只需在安装应用程序时调用一次，因为在数据库中持久创建的表
您需要\xFF\xC2
\xFF\xDA
或相关信息，它会告诉模型它们与哪些数据库连接相关。问题中缺少这个。
答案 1 :(得分：0)
问题似乎已经消失，所以我会发布我的工作。这很简单。我重构了代码以明确地提供类和经典的映射器，而不是使用声明性基础，一切都运行良好......