title: Alembic downgrade doesn't seem to understand the meta data
body: <p>The <strong>model.py</strong> looks like this:</p>
  
  <pre><code>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)
  </code></pre>
  
  <p>I just added <code>is_first_time_news</code> via this alembic migration:</p>
  
  <pre><code>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))
  </code></pre>
  
  <p><code>alembic upgrade head</code> works great.</p>
  
  <p>But when I do a <code>alembic downgrade -1</code> I get a strange exception:</p>
  
  <blockquote>
    <p>AttributeError: Neither 'Column' object nor 'Comparator' object has an
    attribute '_columns'</p>
  </blockquote>