Aggregate join query using Flask-SQLAlchemy and Flask-Marshmallow

I am using Flask-SQLAlchemy and Flask-Marshmallow to pull data from a SQL Server instance. The query that I'm trying to replicate looks like this

SELECT
    o.[ProductId],
    [AccountName] = a.[Name],
    [AccountDescription] = a.[Description],
    [TotalSize] = SUM(d.[Size])
FROM
    [OrderDetail] d
JOIN
    [Order] o
ON
    d.[OrderId] = o.[Id]
JOIN
    [Account] a
ON
    d.[AccountId] = a.[Id]
WHERE
    o.[Timestamp] <= @asOfDate
GROUP BY
    o.[ProductId],
    a.[Name],
    a.[Description]

I am having difficulty translating this to SQLAlchemy, specifically Flask-SQLAlchemy which it seems has a slightly different way of doing things.

This is my attempt, located inside my route function. This query executes, but the SQL that is run against the server is just a join of the three tables—it doesn't do the aggregation or group by. It also is selecting every column, I think, instead of just the ones I've specified in with_entities.

# views.py
@app.route('/product_sizes')
def get_product_sizes() -> Response:
    as_of_date_str = request.args.get('asOfDate')
    query = OrderDetail.query
                       .with_entities(Order.product_id.label('product_id'),
                                      Account.name.label('account_name'),
                                      Account.description.label('account_description'),
                                      func.sum(OrderDetail.size).label('total_size')) \
                       .group_by('product_id', 'account_name', 'account_description')

    if as_of_date_str:
        as_of_date = datetime.strptime(as_of_date_str, '%Y-%m-%d')
        query = query.join(OrderDetail.order).filter(Order.timestamp <= as_of_date)

    result = schemas.product_size_schema.dump(query).data
    json_string = json.dumps(result, cls=DefaultJsonEncoder)
    return Response(response=json_string, status=200, mimetype="application/json")

My questions are

1. How can I fix the query so that it behaves like the SQL query above?
2. Is this roughly the "correct" way of doing things? I am new to SQLAlchemy and Flask, so I'm unsure of the normal way to do things. E.g., giving labels to every column and then grouping by those strings seems a little off—I would have expected to be able to group by the column attributes on my model classes. Also calling join when trying to filter against a joined table seems redundant—isn't that join already encoded in the db.relationship property?

My models and schema are below.

# models.py
class Account(db.Model):
    _id = db.Column('Id', db.Integer, primary_key=True)
    name = db.Column('Name', db.Unicode(250))
    description = db.Column('Description', db.Unicode)

class Order(db.Model):
    _id = db.Column('Id', db.Integer, primary_key=True)
    product_id = db.Column('ProductId', db.Integer)
    timestamp = db.Column('TradeTimestamp', db.DateTime)

class OrderDetail(db.Model):
    _id = db.Column('Id', db.Integer, primary_key=True)
    order_id = db.Column('OrderId', db.ForeignKey('Order.Id'), index=True)
    account_id = db.Column('AccountId', db.ForeignKey('Account.Id'), index=True)
    size = db.Column('Size', db.Numeric(19, 6), nullable=False)

    account = db.relationship('Account', primaryjoin='OrderDetail.account_id == Account._id', backref='order_details', lazy='joined')
    order = db.relationship('Trade', primaryjoin='OrderDetail.order_id == Order._id', backref='order_details', lazy='joined')


"""schemas.py"""
class ProductSizeSchema(ma.ModelSchema):
    """Schema for serializing product size objects"""

    product_id = fields.Int()
    account_name = fields.Str()
    account_description = fields.Str()
    total_size = fields.Decimal(19, 6)

product_size_schema = ProductSizeSchema()
product_sizes_schema = ProductSizeSchema(many=True)

After looking around, I read that it's not at all a big deal to not use the Flask-SQLAlchemy method of querying (e.g., MyModel.query.<something>) if it's not convenient to do so. So I just used db.session.query() in a normal SQLAlchemy fashion.

query = db.session \
          .query(Order.instrument_id,
                 Account.name.label("account_name"),
                 Account.description.label("account_description"),
                 func.sum(OrderDetail.size).label("total_size")) \
          .join(OrderDetail.order) \
          .join(OrderDetail.account) \
          .group_by(Order.instrument_id, Account.name, Account.description)

if as_of_date_str:
    as_of_date = datetime.strptime(as_of_date_str, '%Y-%m-%d')
    query = query.filter(Order.timestamp <= as_of_date)