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SQLAlchemy 1.6 to 2.0 Migration Example

This codemod demonstrates how to use Codegen to automatically migrate SQLAlchemy 1.6 code to the new 2.0-style query interface. For a complete walkthrough, check out our tutorial.

How the Migration Script Works

The codemod script handles four key transformations:

  1. Update Base Class and Imports

    # From:
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()

# To:
from sqlalchemy.orm import DeclarativeBase
class Base(DeclarativeBase):
    pass

    Updates the Base class to use the new DeclarativeBase style.

  2. Modernize ORM Relationships

    # From:
class User(Base):
    addresses = relationship("Address", backref="user")

# To:
class User(Base):
    addresses = relationship("Address", back_populates="user")

    Relationships are modernized to use explicit back_populates instead of backref. If no back reference is specified, it defaults to None.

  3. Update Query Method Names

    # From:
db.session.query(User).filter(User.name == 'john').all()
db.session.query(User).first()

# To:
db.session.select(User).where(User.name == 'john').scalars().all()
db.session.select(User).scalar_one_or_none()

    Updates query method names to their 2.0 equivalents: query → select, filter → where, all → scalars().all, and first → scalar_one_or_none.

  4. Update Configurations

    # From:
create_engine(url)
sessionmaker(bind=engine)
relationship("Address")

# To:
create_engine(url, future=True, pool_pre_ping=True)
sessionmaker(bind=engine, future=True)
relationship("Address", lazy="select")

    Adds future-compatible configurations and default lazy loading settings. Also updates Pydantic configs from orm_mode to from_attributes.

Running the Example

# Install Codegen
pip install codegen

# Run the migration
python run.py

The script will process all Python files in the input_repo directory and apply the transformations in the correct order.

Learn More