flask-marshmallow - generating Marshmallow schemas from SQLAlchemy models seemingly breaks flasgger

[snctfd]

I have a SQLAlchemy model from which I derive a flask-marshmallow schema:

class Customer(db.Model):
    __tablename__ = 'customer'

    customer_id: Mapped[int] = mapped_column('pk_customer_id', primary_key=True)

    first_name: Mapped[str] = mapped_column(String(100))
    last_name: Mapped[str] = mapped_column(String(100))
    customer_number: Mapped[int]

class CustomerSchema(ma.SQLAlchemyAutoSchema):
    class Meta:
        model = Customer

I then add the Schema to my definitions with @swag_from:

@customers_bp.route('/', methods=['GET'])
@swag_from({
    'definitions': {
        'Customer': CustomerSchema
    }
})
def get_customers():
    """
    Return a list of customers. 
    ---
    responses:
    200:
        description: A list of customers
        schema:
            type: object
            properties:
                customers:
                    type: array
                    items:
                        $ref: '#/definitions/Customer'
    """
    ...

When I head to /apidocs, I am greeted with the following error:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/flasgger/base.py", line 164, in get
    return jsonify(self.loader())
  File "/usr/local/lib/python3.9/site-packages/flask/json/__init__.py", line 170, in jsonify
    return current_app.json.response(*args, **kwargs)  # type: ignore[return-value]
  File "/usr/local/lib/python3.9/site-packages/flask/json/provider.py", line 214, in response
    f"{self.dumps(obj, **dump_args)}\n", mimetype=self.mimetype
  File "/usr/local/lib/python3.9/site-packages/flask/json/provider.py", line 179, in dumps
    return json.dumps(obj, **kwargs)
  File "/usr/local/lib/python3.9/json/__init__.py", line 234, in dumps
    return cls(
  File "/usr/local/lib/python3.9/json/encoder.py", line 201, in encode
    chunks = list(chunks)
  File "/usr/local/lib/python3.9/json/encoder.py", line 431, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/usr/local/lib/python3.9/json/encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "/usr/local/lib/python3.9/json/encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "/usr/local/lib/python3.9/json/encoder.py", line 438, in _iterencode
    o = _default(o)
  File "/usr/local/lib/python3.9/site-packages/flask/json/provider.py", line 121, in _default
    raise TypeError(f"Object of type {type(o).__name__} is not JSON serializable")
TypeError: Object of type SQLAlchemyAutoSchemaMeta is not JSON serializable

Seemingly, the Meta object containing options for schema generation causes problems when attempting to include it in the flasgger config.

[superCoderDOM]

We have a similar problem with schemas created from direct imports from flask_marshmallow. The serializer doesn't know what to do with the schema's Meta class:

ERROR:root:jsonify failure; defaulting to json.dumps
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/flasgger/base.py", line 164, in get
    return jsonify(self.loader())
  File "/usr/local/lib/python3.9/site-packages/flask/json/__init__.py", line 342, in jsonify
    return current_app.json.response(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/flask/json/provider.py", line 309, in response
    f"{self.dumps(obj, **dump_args)}\n", mimetype=mimetype
  File "/opt/swept/api/swept_api/__init__.py", line 55, in dumps
    return _json.dumps(obj, cls=ExtendedJSONEncoder, **kwargs)
  File "/usr/local/lib/python3.9/json/__init__.py", line 234, in dumps
    return cls(
  File "/usr/local/lib/python3.9/json/encoder.py", line 201, in encode
    chunks = list(chunks)
  File "/usr/local/lib/python3.9/json/encoder.py", line 431, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/usr/local/lib/python3.9/json/encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "/usr/local/lib/python3.9/json/encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "/usr/local/lib/python3.9/json/encoder.py", line 438, in _iterencode
    o = _default(o)
  File "/opt/swept/api/swept_api/__init__.py", line 50, in default
    return super().default(o)
  File "/usr/local/lib/python3.9/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type SchemaMeta is not JSON serializable

Pipfile:

apispec = "*"
flask = "==2.2.0"
flask-marshmallow = "==0.15.0"
flask-sqlalchemy = "==3.0.2"
marshmallow = "==3.9.0"
marshmallow-sqlalchemy = "==0.24.0"
sqlalchemy = "==2.0.0"

As mentioned in #590, downgrading to flasgger = "==0.9.5" resolves the SchemaMeta issue, but instead raises a different error for us:

Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 1758, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 1734, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "/usr/local/lib/python3.9/site-packages/flask/views.py", line 107, in view
    return current_app.ensure_sync(self.dispatch_request)(**kwargs)
  File "/usr/local/lib/python3.9/site-packages/flask/views.py", line 188, in dispatch_request
    return current_app.ensure_sync(meth)(**kwargs)
  File "/usr/local/lib/python3.9/site-packages/flasgger/base.py", line 133, in get
    return jsonify(self.loader())
  File "/usr/local/lib/python3.9/site-packages/flasgger/base.py", line 399, in get_apispecs
    specs = get_specs(
  File "/usr/local/lib/python3.9/site-packages/flasgger/utils.py", line 134, in get_specs
    convert_schemas(apispec_swag, apispec_definitions)
  File "/usr/local/lib/python3.9/site-packages/flasgger/marshmallow_apispec.py", line 109, in convert_schemas
    v = convert_schemas(v, definitions)
  File "/usr/local/lib/python3.9/site-packages/flasgger/marshmallow_apispec.py", line 123, in convert_schemas
    definitions[v.__name__] = schema2jsonschema(v)
  File "/usr/local/lib/python3.9/site-packages/apispec/ext/marshmallow/openapi.py", line 256, in schema2jsonschema
    jsonschema = self.fields2jsonschema(fields, partial=partial)
  File "/usr/local/lib/python3.9/site-packages/apispec/ext/marshmallow/openapi.py", line 281, in fields2jsonschema
    prop = self.field2property(field_obj)
  File "/usr/local/lib/python3.9/site-packages/apispec/ext/marshmallow/field_converter.py", line 174, in field2property
    ret.update(attr_func(field, ret=ret))
  File "/usr/local/lib/python3.9/site-packages/apispec/ext/marshmallow/field_converter.py", line 224, in field2default
    default = field.load_default
AttributeError: 'String' object has no attribute 'load_default'

The only schema currently attached to a view is extremely simple:

from flasgger import Schema, fields


class InviteUserSchema(Schema):
    class Meta:
        strict = True

    message = fields.Str(required=True)
    subject = fields.Str(required=True)

In the view extended from SwaggerView, we pass the schema to definitions. If this line is commented out, the docs load without the schema definition, otherwise they raise the error above:

definitions = {"InviteUserSchema": InviteUserSchema}