websauna admin does not detect audiostream table

models.py

"""Place your SQLAlchemy models in this file."""

import datetime
from uuid import uuid4

from sqlalchemy import Column, Unicode, String, Integer, ForeignKey, UnicodeText
from sqlalchemy.dialects.postgresql import UUID
from sqlalchemy.orm import relationship

from websauna.system.model.meta import Base
from websauna.system.model.columns import UTCDateTime
from websauna.utils.time import now

class Media(Base):

    #: The table in the database
    __tablename__ = "media"

    #: Database primary key for the row (running counter)
    id = Column(Integer, autoincrement=True, primary_key=True)

    #: Publicly exposed non-guessable
    uuid = Column(UUID(as_uuid=True), default=uuid4)

    #: url
    url = Column(Unicode(384), default=None)

    #: title of content
    title = Column(Unicode(384), default=None)

    #: duration of content
    duration = Column(String(8), default=None)
    
    #: description of content
    description = Column(UnicodeText, default=None)

    #: When this content was published
    published_at = Column(UTCDateTime, default=None)

    #: Relationship mapping between media and audiostream.
    #: Each audiostream can have only media.
    #: Deleting media deletes its audiostreams.
    audiostreams = relationship("AudioStream",
                           back_populates="media",
                           lazy="dynamic",
                           cascade="all, delete-orphan",
                           single_parent=True)

    def is_recent(self):
        return self.published_at >= now() - datetime.timedelta(days=1)

    def __repr__(self):
        return "#{}: {}".format(self.id, self.url)

    def __str__(self):
        """Python default and admin UI string presentation."""
        return self.url


class AudioStream(Base):

    #: The table in the database
    __tablename__ = "audiostream"

    #: Database primary key for the row (running counter)
    id = Column(Integer, autoincrement=True, primary_key=True)

    #: Publicly exposed non-guessable id
    uuid = Column(UUID(as_uuid=True), default=uuid4)

    #: What the user sees for this audiostream
    filepath = Column(Unicode(512), default=None)
    bitrate = Column(String(4), default=None)
    extension = Column(String(4), default=None)
    
    #: download url
#     url = Column(Unicode(768), default=None)

    #: Which question this choice is part of
    media_id = Column(Integer, ForeignKey('media.id'))
    media = relationship("Media", back_populates="audiostreams")

    def __repr__(self):
        """Shell and debugger presentation."""
        return "#{}@{}".format(self.extension, self.bitrate)

    def __str__(self):
        """Python default and admin UI string presentation."""
        return "#{}@{}".format(self.extension, self.bitrate)