Downgrade assets db by adding a not null constraint on
    ``equities.first_traded``

def _downgrade_v3(op):
    """
    Downgrade assets db by adding a not null constraint on
    ``equities.first_traded``
    """
    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'],
    )