python - SQLAlchemy 给出 "MetaData not bound to Engine or Connection"

Question

我有这段代码来创建一个数据库:

from sqlalchemy import create_engine
from sqlalchemy import Table, Column, Integer, String, Float, MetaData, ForeignKey
from sqlalchemy.sql import select, and_
from PyQt4 import QtGui, QtCore

class DbUtils(object):
    def __init__(self, db_file = None, parent = None):

        self.db = None
        self.db_connection = None
        self.db_file = str(db_file)

    def db_open(self):
        self.db = create_engine('sqlite:///' + self.db_file)
        self.db_connection = self.db.connect()

    def db_close(self):
        self.db_connection.close()

    def db_create_voltdrop(self):
        metadata = MetaData()

        tb_cable_brands = Table('cable_brands', metadata,
            Column('id', Integer, primary_key=True),
            Column('brand', String)
            )
        tb_cable_types = Table('cable_types', metadata,
            Column('id', Integer, primary_key=True),
            Column('brand_id', None, ForeignKey('cable_brands.id')),
            Column('type', String),
            Column('alpha', String)
            )
        tb_cable_data = Table('cable_data', metadata,
            Column('id', Integer, primary_key=True),
            Column('type_id', None, ForeignKey('cable_types.id')),
            Column('size', String),
            Column('resistance', Float)
            )
        metadata.create_all(self.db) # db_utils.py", line 67

当它是我的 GUI 的 QMainWindow 类的一部分时，它工作正常，没有错误。但是当我将它从 GUI 分离到一个单独的模块时，它开始给我那个错误。这是一个回溯:

Traceback (most recent call last):
File "C:\Temp\xxx\applications\voltdrop\mainwindow.py", line 52, in __init__ 
    self.main_ui()
File "C:\Temp\xxx\applications\voltdrop\mainwindow.py", line 212, in main_ui 
    self.db_utils.db_create_voltdrop()
File "C:\Temp\xxx\scripts\db_utils.py", line 67, in db_create_voltdrop 
    metadata.create_all(self.db)
File "C:\Temp\PortablePython\App\lib\site-packages\sqlalchemy\schema.py", line 2511, in create_all 
    bind = _bind_or_error(self)
File "C:\Temp\PortablePython\App\lib\site-packages\sqlalchemy\schema.py", line 3124, in _bind_or_error 
    raise exc.UnboundExecutionError(msg)

sqlalchemy.exc.UnboundExecutionError: 
The MetaData is not bound to an Engine or Connection.  
Execution can not proceed without a database to execute against.  
Either execute with an explicit connection or assign the MetaData's .bind to enable implicit execution.


    self.db_utils = DbUtils(self.cabledata_db, self)
    self.main_ui() # mainwindow.py", line 52

def main_ui(self):
...
    if not self.find_data_file(self.cabledata_db):
        file = self.select_data_file("cable")
        if file:
            self.cabledata_db = file
            self.db_utils.db_open()
        else:
            self.db_utils.db_create_voltdrop() # mainwindow.py", line 212

我做错了什么？谢谢。

[已解决]

谢谢。它是这样工作的:如果没有数据库文件，打开对话框并选择一个，如果对话框被取消，则创建一个空数据库。如果对话框选择了一个文件，则使用该数据库。否则，使用找到的数据库文件。

这现在有效:

    if not self.find_data_file(self.cabledata_db):
        file = self.select_data_file("cable")
        if file:
            self.cabledata_db = file
            self.db_utils.db_open()
        else:
            self.db_utils.db_open()
            self.db_utils.db_create_voltdrop()
    else:
        self.db_utils.db_open()

Answer 1

如果不是文件，则在调用 db_utils.db_create_voltdrop() 之前不调用 db_utils.db_open()。

因此，当调用 metadata.create_all(self.db) 时，db_utils.db 的值为 None。确保在调用 db_utils.db_create_voltdrop() 之前调用 db_utils.db_open() 应该可以解决您的问题。

metadata.create_all(None) 引发 sqlalchemy.exc.UnboundExecutionError 而不是 ValueError 的原因是它们允许您这样做:

metadata.bind = self.db
metadata.create_all()