Alembic downgrade doesn't seem to understand the meta data

Question

The model.py looks like this:

import datetime

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, Numeric, ForeignKey, DateTime, Boolean
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, relationship

from configs import config_base as config
Base = declarative_base()


class User(Base):
    __tablename__ = 'user'

    id = Column(String, unique=True, primary_key=True)
    name = Column(String(100), nullable=False)
    team_id = Column(String, ForeignKey('team.id'))
    last_modified_on = Column(DateTime, default=datetime.datetime.utcnow())
    team = relationship('Team', back_populates='members')


class Team(Base):
    __tablename__ = 'team'

    id = Column(String, unique=True, primary_key=True)
    name = Column(String, nullable=False)
    bot_access_token = Column(String(100), nullable=False)
    bot_user_id = Column(String(100), nullable=False)
    last_modified_on = Column(DateTime, default=datetime.datetime.utcnow())
    is_active = Column(Boolean, default=True)
    members = relationship('User', back_populates='team')
    is_first_time_news = Column(Boolean, default=True)

engine = create_engine(config.SQLALCHEMY_DATABASE_URI)
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)

I just added is_first_time_news via this alembic migration:

revision = '6f9e2d360276'
down_revision = None
branch_labels = None
depends_on = None

from alembic import op
import sqlalchemy as sa


def upgrade():
    op.add_column('team', sa.Column('is_first_time_news', sa.Boolean, default=False))


def downgrade():
    op.drop_column('team', sa.Column('is_first_time_news', sa.Boolean))

alembic upgrade head works great.

But when I do a alembic downgrade -1 I get a strange exception:

AttributeError: Neither 'Column' object nor 'Comparator' object has an attribute '_columns'

Answer 1

Are you using sqlite? Sqlite does not allow you to drop a column from the scheme. I had a similar problem when I tried to downgrade a local sqlite database I was testing.

SQLite supports a limited subset of ALTER TABLE. The ALTER TABLE command in SQLite allows the user to rename a table or to add a new column to an existing table.

https://www.sqlite.org/lang_altertable.html