SQLAlchemy: Join table with object's children - sqlalchemy

Question

I have three object types with corresponding tables:
class Order:
suborders = relationship('Suborder', lazy='dynamic')
class Suborder:
...
class PurchaseOrder:
suborder = relationship('Suborder', foreign_keys=[suborder_id], lazy='joined')
Now I need to get all PurchaseOrder instances matching Order's suborders. In pure SQL I'd write something like this:
SELECT po.*
FROM purchase_orders AS po JOIN suborders AS so ON po.suborder_id = so.id
WHERE so.order_id = 'order-007'
How do I do it using SQLAlchemy? I tried this:
o = Order.query.get('order-007')
PurchaseOrder.query.join(o.suborders)
But this gave me an error:
AttributeError: 'AppenderQuery' object has no attribute 'is_selectable'
What is the right way to do it?

Answer 1

Getting required data with just one query:
query = (
PurchaseOrder.query
.join(Suborder)
.filter(Suborder.order_id == 'order-007')
)
If you already have the Order instance o, you can do the following:
o = Order.query.get('order-007') # already have the order instance
query = (
session.query(PurchaseOrder)
.join(Suborder)
.with_parent(o)
)
, but you will still need to join on Suborder.
But again, the first one would correspond to the SQL query you provided as the SQL implementation.