def serialize(obj, *, fields=None, expand=None):
    expand = set(expand or [])
    fields = set(fields or [])
    out = {}
    # base columns
    for col in obj.__table__.columns:
        name = col.key
        if fields and name not in fields:
            continue
        out[name] = getattr(obj, name)
    # expansions
    for rel in obj.__mapper__.relationships:
        if rel.key not in expand:
            continue
        val = getattr(obj, rel.key)
        if val is None:
            out[rel.key] = None
        elif rel.uselist:
            out[rel.key] = [serialize(child) for child in val]
        else:
            out[rel.key] = serialize(val)
    return out