Modify data as part of an alembic upgrade

I would like to modify some database data as part of an alembic upgrade.

I thought I could just add any code in the upgrade of my migration, but the following fails:

def upgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.add_column('smsdelivery', sa.Column('sms_message_part_id', sa.Integer(), sa.ForeignKey('smsmessagepart.id'), nullable=True))
    ### end Alembic commands ###

    from volunteer.models import DBSession, SmsDelivery, SmsMessagePart

    for sms_delivery in DBSession.query(SmsDelivery).all():
        message_part = DBSession.query(SmsMessagePart).filter(SmsMessagePart.message_id == sms_delivery.message_id).first()
        if message_part is not None:
            sms_delivery.sms_message_part = message_part

with the following error:

sqlalchemy.exc.UnboundExecutionError: Could not locate a bind configured on mapper Mapper|SmsDelivery|smsdelivery, SQL expression or this Session

I am not really understanding this error. How can I fix this or is doing operations like this not a possibility?