[Sqlalchemy-tickets] Issue #3389: ORM relationship backref missing unless manually call configure_mappers() (zzzeek/sqlalchemy)

[Marcus C. <iss...@bi...>]

New issue 3389: ORM relationship backref missing unless manually call configure_mappers()
https://bitbucket.org/zzzeek/sqlalchemy/issue/3389/orm-relationship-backref-missing-unless

Marcus Cobden:

This is with SQLAlchemy 1.0.2


```
#!python

import logging
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, ForeignKey, Integer, String
from sqlalchemy.orm import relationship, configure_mappers
 
 
logging.basicConfig(level=logging.DEBUG)
Base = declarative_base()
 
 
class Project(Base):
    __tablename__ = 'projects'
    # Internal database ID
    _id = Column(Integer, primary_key=True)
    # API id
    slug = Column(String, unique=True)
    runs = relationship(
        'Run',
        backref='project',
        cascade="all, delete-orphan")
 
class Run(Base):
    __tablename__ = 'runs'
    _id = Column(Integer, primary_key=True)
    _project_id = Column(Integer, ForeignKey('projects._id'), nullable=False)
 
    number = Column(Integer, unique=True, autoincrement=True)
 
 
# configure_mappers()
 
 
print Project.runs # works 'Project.runs'
print Run.project # Attribute Error!
```