code: def _downgrade_v3(op):
      
      op.create_table(
          '_new_equities',
          sa.Column(
              'sid',
              sa.Integer,
              unique=True,
              nullable=False,
              primary_key=True,
          ),
          sa.Column('symbol', sa.Text),
          sa.Column('company_symbol', sa.Text),
          sa.Column('share_class_symbol', sa.Text),
          sa.Column('fuzzy_symbol', sa.Text),
          sa.Column('asset_name', sa.Text),
          sa.Column('start_date', sa.Integer, default=0, nullable=False),
          sa.Column('end_date', sa.Integer, nullable=False),
          sa.Column('first_traded', sa.Integer, nullable=False),
          sa.Column('auto_close_date', sa.Integer),
          sa.Column('exchange', sa.Text),
      )
      op.execute(
          """
          insert into _new_equities
          select * from equities
          where equities.first_traded is not null
          """,
      )
      op.drop_table('equities')
      op.rename_table('_new_equities', 'equities')
      # we need to make sure the indices have the proper names after the rename
      op.create_index(
          'ix_equities_company_symbol',
          'equities',
          ['company_symbol'],
      )
      op.create_index(
          'ix_equities_fuzzy_symbol',
          'equities',
          ['fuzzy_symbol'],
      )
docs: Downgrade assets db by adding a not null constraint on
      ``equities.first_traded``