The Setup
I’m writing a web app in python with
Flask and
SqlAlchemy with MySQL.
the solution…
The Code
Starting with the following (slightly snipped for brevity) python/SqlAlchemy code in
<project>/markbox/models.py:
class UserBlog(ModelBase):
__tablename__ = 'user_blog'
id = sa.Column(sa.Integer, primary_key=True)
uid = sa.Column(strcol)
# blog title
name = sa.Column(strcol)
# returns a query => blog.sync_stats.all()
sync_stats = relationship(
'models.SyncStats', backref='blog',
lazy='dynamic')
class SyncStats(ModelBase):
"""
Store various statistics about sync operations
"""
__tablename__ = 'system_syncstats'
id = sa.Column(sa.Integer, primary_key=True)
uid = sa.Column(strcol, sa.ForeignKey('user_blogsettings.uid'))
And this code in
<project>/markbox/tools/test.py:
import sys
import os
sys.path.append(os.path.abspath('.'))
sys.path.append(os.path.abspath('markbox'))
print sys.path
from markbox.models import UserBlog, SyncStats
from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, DateTime
from sqlalchemy.orm import sessionmaker
SQLALCHEMY_DATABASE_URI = "mysql://markbox_user:markbox@localhost/markbox"
engine = create_engine(SQLALCHEMY_DATABASE_URI, echo='debug')
Session = sessionmaker(bind=engine)
session = Session()
if __name__ == '__main__':
blog = UserBlog.query.limit(1)
The Error
When run from the command line in <project>:
% python markbox/tools/test.py
I get:
['/Users/sivy/Projects/personal/markbox/markbox/tools', ..., '/Users/sivy/Projects/personal/markbox', '/Users/sivy/Projects/personal/markbox/markbox']
Traceback (most recent call last):
File "markbox/tools/test.py", line 25, in <module>
blog = UserBlog.query.limit(1)
File "/Users/sivy/.virtualenvs/markbox/lib/python2.7/site-packages/sqlalchemy/orm/scoping.py", line 131, in __get__
mapper = class_mapper(owner)
File "/Users/sivy/.virtualenvs/markbox/lib/python2.7/site-packages/sqlalchemy/orm/util.py", line 1112, in class_mapper
mapper = _inspect_mapped_class(class_, configure=configure)
File "/Users/sivy/.virtualenvs/markbox/lib/python2.7/site-packages/sqlalchemy/orm/util.py", line 1045, in _inspect_mapped_class
mapperlib.configure_mappers()
File "/Users/sivy/.virtualenvs/markbox/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 2122, in configure_mappers
mapper._post_configure_properties()
File "/Users/sivy/.virtualenvs/markbox/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py", line 1244, in _post_configure_properties
prop.init()
File "/Users/sivy/.virtualenvs/markbox/lib/python2.7/site-packages/sqlalchemy/orm/interfaces.py", line 231, in init
self.do_init()
File "/Users/sivy/.virtualenvs/markbox/lib/python2.7/site-packages/sqlalchemy/orm/properties.py", line 1031, in do_init
self._generate_backref()
File "/Users/sivy/.virtualenvs/markbox/lib/python2.7/site-packages/sqlalchemy/orm/properties.py", line 1220, in _generate_backref
self, m))
sqlalchemy.exc.ArgumentError: Error creating backref 'blog' on relationship 'UserBlog.sync_stats': property of that name exists on mapper 'Mapper|SyncStats|system_syncstats'
What I’ve Done
I’ve already confirmed that:
- the SyncStats model has no “blog” property (it used to have a method, decorated with @property).
- I’ve deleted any existing *.pyc files to make sure that the interpreter is not picking up pre-compiled code.
The solution
So, the problem turned out to be that the models were being imported from two paths:
from models import UserBlog
and later:
from markbox.models import UserBlog
This caused the intilization code to be run twice, and the resulting error.
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