How to use marshmallow-sqlalchemy with async code? - sqlalchemy

Question

I'm trying to use marshmallow-sqlalchemy with aiohttp and I have followed their docs with the basic example and I'm getting an error.
I have this schema:
from marshmallow_sqlalchemy import SQLAlchemyAutoSchema
from db.customer import Customer
class CustomerSchema(SQLAlchemyAutoSchema):
class Meta:
model = Customer
include_relationships = True
load_instance = True
And then the following code for the query:
from sqlalchemy import select
from db import db_conn
from db.customer import Customer
from queries.schema import CustomerSchema
customer_schema = CustomerSchema()
async def get_all_users():
async with db_conn.get_async_sa_session() as session:
statement = select(Customer)
results = await session.execute(statement)
_ = (results.scalars().all())
print(_)
response = customer_schema.dump(_, many=True)
print(response)
For the first print statement I'm getting
[<db.customer.Customer object at 0x10a183340>, <db.customer.Customer object at 0x10a183940>, <db.customer.Customer object at 0x10b0cd9d0>]
But then it fails with
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/util/_concurrency_py3k.py", line 60, in await_only
raise exc.MissingGreenlet(
sqlalchemy.exc.MissingGreenlet: greenlet_spawn has not been called; can't call await_() here. Was IO attempted in an unexpected place? (Background on this error at: http://sqlalche.me/e/14/xd2s)
So how can I use marshmallow-sqlalchemy to serialize the SqlAlchemy reponse?
Another options (packages, etc) or a generic custom solutions are OK too.
For the time being I'm using this:
statement = select(Customer)
results = await session.execute(statement)
_ = (results.scalars().all())
response = {}
for result in _:
value = {k: (v if not isinstance(v, sqlalchemy.orm.state.InstanceState) else '_') for k, v in result.__dict__.items()}
response[f'customer {value["id"]}'] = value
return response
Full traceback:
Traceback (most recent call last):
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/aiohttp/web_protocol.py", line 422, in _handle_request
resp = await self._request_handler(request)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/aiohttp/web_app.py", line 499, in _handle
resp = await handler(request)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/aiohttp/web_urldispatcher.py", line 948, in _iter
resp = await method()
File "/Users/ruslan/OneDrive/Home/Dev/projects/code/education/other/cft/views/user.py", line 24, in get
await get_all_users()
File "/Users/ruslan/OneDrive/Home/Dev/projects/code/education/other/cft/queries/user.py", line 18, in get_all_users
response = customer_schema.dump(_, many=True)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/marshmallow/schema.py", line 547, in dump
result = self._serialize(processed_obj, many=many)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/marshmallow/schema.py", line 509, in _serialize
return [
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/marshmallow/schema.py", line 510, in <listcomp>
self._serialize(d, many=False)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/marshmallow/schema.py", line 515, in _serialize
value = field_obj.serialize(attr_name, obj, accessor=self.get_attribute)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/marshmallow/fields.py", line 310, in serialize
value = self.get_value(obj, attr, accessor=accessor)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/marshmallow_sqlalchemy/fields.py", line 27, in get_value
return super(fields.List, self).get_value(obj, attr, accessor=accessor)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/marshmallow/fields.py", line 239, in get_value
return accessor_func(obj, check_key, default)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/marshmallow/schema.py", line 472, in get_attribute
return get_value(obj, attr, default)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/marshmallow/utils.py", line 239, in get_value
return _get_value_for_key(obj, key, default)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/marshmallow/utils.py", line 253, in _get_value_for_key
return getattr(obj, key, default)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/orm/attributes.py", line 480, in __get__
return self.impl.get(state, dict_)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/orm/attributes.py", line 931, in get
value = self.callable_(state, passive)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/orm/strategies.py", line 879, in _load_for_state
return self._emit_lazyload(
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/orm/strategies.py", line 1036, in _emit_lazyload
result = session.execute(
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 1689, in execute
result = conn._execute_20(statement, params or {}, execution_options)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1582, in _execute_20
return meth(self, args_10style, kwargs_10style, execution_options)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/sql/lambdas.py", line 481, in _execute_on_connection
return connection._execute_clauseelement(
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1451, in _execute_clauseelement
ret = self._execute_context(
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1813, in _execute_context
self._handle_dbapi_exception(
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1998, in _handle_dbapi_exception
util.raise_(exc_info[1], with_traceback=exc_info[2])
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/util/compat.py", line 207, in raise_
raise exception
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1770, in _execute_context
self.dialect.do_execute(
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 717, in do_execute
cursor.execute(statement, parameters)
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/dialects/postgresql/asyncpg.py", line 449, in execute
self._adapt_connection.await_(
File "/Users/ruslan/.local/share/virtualenvs/cft-RKlbQ9iX/lib/python3.9/site-packages/sqlalchemy/util/_concurrency_py3k.py", line 60, in await_only
raise exc.MissingGreenlet(
sqlalchemy.exc.MissingGreenlet: greenlet_spawn has not been called; can't call await_() here. Was IO attempted in an unexpected place? (Background on this error at: http://sqlalche.me/e/14/xd2s)

Answer 1

The problem in this case is that the Marshmallow schema is configured to load related models (include_relationships=True). Since the initial query doesn't load them automatically, the schema triggers a query to fetch them, and this causes the error.
The simplest solution, demonstrated in the docs, is to eagerly load the related objects with their "parent":
async def get_all_users():
async with db_conn.get_async_sa_session() as session:
# Let's assume a Customer has a 1 to many relationship with an Order model
statement = select(Customer).options(orm.selectinload(Customer.orders))
results = await session.execute(statement)
_ = (results.scalars().all())
print(_)
response = customer_schema.dump(_, many=True)
print(response)
There is more discussion in the Preventing Implicit IO when Using AsyncSession section of the docs.