Backref relationships don't populate in the class until instance is created

[jinschoi]

Describe the bug

Referencing a backref relationship raises an AttributeError unless an instance is instantiated first.

This causes a problem when the first thing you do is try to use a 2.0-style select() query using one of the relationships in a condition. It is easy enough to workaround, but the relationship should exist at the start.

To Reproduce

from sqlalchemy import *
from sqlalchemy.orm import *

Base = declarative_base()

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship('Child', backref='parent')

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parentid = Column(Integer, ForeignKey('parent.id'))

# Uncommenting this makes the relationship available.
# c = Child()

Child.parent

Error

>>> Traceback (most recent call last):
>>> Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../test.py", line 19, in <module>
    Child.parent
AttributeError: type object 'Child' has no attribute 'parent'

Versions

• OS: macOS 12.0.1
• Python: 3.8.12
• SQLAlchemy: 1.4.23
• Database: all
• DBAPI (eg: psycopg, cx_oracle, mysqlclient): all

Additional context

No response

[CaselIT]

Hi,

This seems due to the lazy initialization of the modules. You should place a call to configure_mappers() one they are all imported. I don't think there is much we can do without breaking other use cases here.

The suggested alternative is to avoid backref and use instead backpopulate with an explicit relationship.