flask-sqlalchemy: polymorphic and python mixin

model.py

class Asset(db.Model): 
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(120), nullable=False)

    asset_type_id = db.Column(db.Integer(), db.ForeignKey('asset_type.id'), nullable=False)
    asset_type = db.relationship('AssetType', backref='assets')

    # I would like a nicer way to do this
    @property
    def versions(self):
        if self.asset_type.name=='video':
            return self.asset_video_versions
        elif self.asset_type.name=='image':
            return self.asset_image_versions
        else: # =='file'
            return self.asset_file_versions

    def __str__(self):
        return self.name



class AssetType(db.Model):
    id = db.Column(db.Integer, primary_key = True)
    name = db.Column(db.String(120), nullable=False)

    def __str__(self):
        return self.name


class AssetVersionMixin(object):
    @declared_attr
    def __tablename__(cls):
        return re.sub('(?!^)([A-Z]+)', r'_\1', cls.__name__).lower()

    id = db.Column(db.Integer, primary_key=True)
    filetype = db.Column(db.String(7), nullable=False)
    size = db.Column(db.BigInteger)
    @declared_attr
    def asset_id(cls): return db.Column(db.Integer, db.ForeignKey('asset.id'))

    # can this be just 'versions'?
    @declared_attr
    def asset(cls): return db.relationship(Asset, backref=cls.__tablename__+'_versions')

    @property
    def direct_link(self):
        return self.asset.link +'.'+ self.filetype

    @property
    def serialize(self):
        return {
            'id':self.id,
            'size':self.size,
            'filetype':self.filetype,
            'link':self.direct_link
        }

    def __str__(self):
        return '%s %s' % (cls.__tablename__, self.id,)


class AssetFile(db.Model, AssetVersionMixin):
    @property
    def serialize(self):
        return super(AssetFile, self).serialize


class AssetImage(db.Model, AssetVersionMixin):
    res_x = db.Column(db.Integer)
    res_y = db.Column(db.Integer)

    @property
    def serialize(self):
        property_dict = super(AssetImage, self).serialize
        property_dict.update({'res_x':self.res_x, 'res_y':self.res_y})
        return property_dict


class AssetVideo(db.Model, AssetVersionMixin):
    res_x = db.Column(db.Integer)
    res_y = db.Column(db.Integer)
    duration = db.Column(db.Integer)
    format = db.Column(db.String(15))

    @property
    def serialize(self):
        property_dict = super(AssetVideo, self).serialize
        property_dict.update({'res_x':self.res_x, 'res_y':self.res_y, 'duration':self.duration, 'format':self.format})
        return property_dict