Upgrade database.

def upgrade():
    """Upgrade database."""
    op.drop_constraint(u'fk_access_actionsusers_user_id_accounts_user',
                       'access_actionsusers', type_='foreignkey')
    op.drop_index(op.f('ix_access_actionsusers_user_id'),
                  table_name='access_actionsusers')

    op.alter_column('access_actionsusers', 'user_id', nullable=False,
                    existing_type=sa.Integer())

    op.create_index(op.f('ix_access_actionsusers_user_id'),
                    'access_actionsusers', ['user_id'], unique=False)
    op.create_foreign_key(op.f('fk_access_actionsusers_user_id_accounts_user'),
                          'access_actionsusers', 'accounts_user', ['user_id'],
                          ['id'], ondelete='CASCADE')