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我一直在試圖在後端使用SQLAlchemy來獲得體面大小的項目。我有跨多個文件的表模型，它自己的文件中有一個聲明基，還有一個幫助文件來包裝常用的SQLAlchemy函數和驅動程序文件。SQLAlchemy從ORM實例化對象時出現AttributeError失敗：mapper
我正在上傳數據，然後決定添加一列。由於這只是測試數據，我認爲最簡單的方法是刪除所有表並重新開始......然後，當我嘗試重新創建模式和表時，常用的聲明式基類突然有空元數據。我通過導入類聲明文件解決了這個問題 - 奇怪，因爲我之前不需要這些導入 - 並且能夠成功重新創建模式。
但現在，當我再次嘗試創建對象，我得到一個錯誤：
AttributeError: mapper
現在我完全糊塗了！有人可以解釋這裏發生了什麼嗎？在我放棄架構之前它工作正常，現在我無法正常工作。
這是我設置的骨架：
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
)
堆棧跟蹤：（出於完整性）
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
我知道這篇文章很長，但我想給完整的圖片。首先，我很困惑爲什麼base.Base模式首先是空的。現在我很困惑爲什麼Categories對象缺少映射器！
任何幫助/見解/建議非常感謝，謝謝！
編輯：
所以模型文件和helper.py處於supackage和driver.py實際上是在同級子包一個文件，它的代碼被包裝在一個函數。該驅動程序功能由包級主文件調用。所以我不認爲這可能是因爲SQLAlchemy沒有時間來初始化？ （如果我正確地理解了答案），這裏是主文件（的相關部分）的樣子：
main。潘岳：
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實際上看起來像：
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
編輯2： 我開始懷疑潛在的問題是一樣的道理我突然需要導入所有模型以在helper.py中設置架構。如果我打印導入的模型對象的表，他們沒有約束的元數據或模式：
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))}
我不知道爲什麼，從創建數據庫基礎中的元數據是不是gettng約束？
是否在Category Category中的此行出現錯字或粘貼錯誤？ '__tablename__ ='business'' – brandaemon
是的，我已經修復了 – Taaam
[Python SqlAlchemy - AttributeError：mapper]的可能重複（https://stackoverflow.com/questions/45534903/python-sqlalchemy-attributeerror-mapper） –