I have two classes, Tag and Hardware, defined with a simple parent-child relationship (see the full definition at the end).

Now I want to filter a query on Tag using the version field in Hardware through an attribute_mapped_collection, eg:

def get_tags(order_code=None, hardware_filters=None):
    session = Session()
    query = session.query(Tag)
    if order_code:
        query = query.filter(Tag.order_code == order_code)
    if hardware_filters:
        for k, v in hardware_filters.iteritems():
            query = query.filter(getattr(Tag.hardware, k).version == v)
    return query.all()

But I get:

AttributeError: Neither 'InstrumentedAttribute' object nor 'Comparator' object associated with Tag.hardware has an attribute 'baseband

The same thing happens if I strip it back by hard-coding the attribute, eg:

query.filter(Tag.hardware.baseband.version == v)

I can do it this way:

query = query.filter(Tag.hardware.any(artefact=k, version=v))

But why can't I filter directly through the attribute?

Class definitions

class Tag(Base):
    __tablename__ = 'tag'
    tag_id = Column(Integer, primary_key=True)
    order_code = Column(String, nullable=False)
    version = Column(String, nullable=False)
    status = Column(String, nullable=False)
    comments = Column(String)
    hardware = relationship(
        "Hardware",
        backref="tag",
        collection_class=attribute_mapped_collection('artefact'),
    )
    __table_args__ = (
        UniqueConstraint('order_code', 'version'),
    )

class Hardware(Base):
    __tablename__ = 'hardware'
    hardware_id = Column(Integer, primary_key=True)
    tag_id = Column(String, ForeignKey('tag.tag_id'))
    product_id = Column(String, nullable=True)
    artefact = Column(String, nullable=False)
    version = Column(String, nullable=False)