release_id = 2

from nailgun.db import db
from nailgun.db.sqlalchemy import models

rel = db().query(models.Release).filter_by(id=release_id).first()
meta = rel.volumes_metadata
volumes = meta['volumes']
for v_id in xrange(len(volumes)):
    volume = volumes[v_id]
    if volume['min_size']['generator'] == 'calc_min_log_size':
        volumes[v_id]['min_size']['generator'] = 'calc_gb_to_mb'
        volumes[v_id]['min_size']['generator_args'] = [2]


db().query(models.Release).filter_by(id=2).update({"volumes_metadata": meta})
db().commit()
db().flush()