AttributeError when de/serializing models with DateTime columns.

[kiptoomm]

I am stumped by this error that arises when de/serializing a model that defines sqlalchemy.DateTime fields:

AttributeError: 'unicode' object has no attribute 'name'

Here is the skeleton of my project:

... 
from sqlalchemy.ext.declarative import declared_attr
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy()
... 

# models.py
class Gender(enum.IntEnum):
    MALE = 0
    FEMALE = 1
    UNKNOWN = 2

class Author(db.Model):
    @declared_attr
    def __tablename__(cls):
        # API endpoint will take the form '/api/__tablename__'
        return cls.__name__.lower()

    id = Column(Integer, primary_key=True)
    user_name = Column(String(64))
    gender = db.Column(db.Enum(Gender), default=Gender.UNKNOWN)
    # timestamps
    created_at = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
    updated_at = db.Column(db.DateTime, onupdate=datetime.utcnow)
... 

# schemas.py
class AuthorSchema(BaseSchema):
    user_name = fields.Str()
    gender = EnumField(Gender)

    class Meta(BaseSchema.Meta):
        type_ = 'author'
        model = Author
...

# de/serializers.py
class AuthorSerializer(DefaultSerializer):
    def serialize(self, instance, only=None):
        schema = AuthorSchema(only=only)
        return schema.dump(instance).data

    def serialize_many(self, instances, only=None):
        schema = AuthorSchema(many=True, only=only)
        return schema.dump(instances).data
# AuthorDeSerializer almost identical to AuthorSerializer except with schema.load()

I see this error when making PATCH requests against EnumFields (using flask-restless to simplify API creation). PATCHing non-enum fields of the model works just fine.

The error does not occur when I remove the timestamp fields from the Author model. I'm not sure why the presence of a DateTime would impact the EnumField serialization. Any idea?

[justanr]

Got a stack trace I can take a look at?

[kiptoomm]

Yup, here it is:

127.0.0.1 - - [17/Jan/2018 17:58:27] "PATCH /api/author1 HTTP/1.1" 500 -
Traceback (most recent call last):
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask/app.py", line 1997, in __call__
    return self.wsgi_app(environ, start_response)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask/app.py", line 1985, in wsgi_app
    response = self.handle_exception(e)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask/app.py", line 1540, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask/app.py", line 1982, in wsgi_app
    response = self.full_dispatch_request()
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask/app.py", line 1614, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask/app.py", line 1517, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask_restless/views/base.py", line 416, in new_func
    return func(*args, **kw)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask_restless/views/base.py", line 364, in new_func
    return func(*args, **kw)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask_restless/views/base.py", line 250, in new_func
    return func(*args, **kw)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask/views.py", line 84, in view
    return self.dispatch_request(*args, **kwargs)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask/views.py", line 149, in dispatch_request
    return meth(*args, **kwargs)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask_restless/views/base.py", line 450, in wrapped
    return func(*args, **kw)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/flask_restless/views/resources.py", line 721, in patch
    result = self.serializer.serialize(instance, only=only)
  File "~/dev/restless_projectsafiria/schema.py", line 108, in serialize
    return schema.dump(instance).data
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/marshmallow/schema.py", line 513, in dump
    **kwargs
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/marshmallow/marshalling.py", line 147, in serialize
    index=(index if index_errors else None)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/marshmallow/marshalling.py", line 68, in call_and_store
    value = getter_func(data)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/marshmallow/marshalling.py", line 141, in <lambda>
    getter = lambda d: field_obj.serialize(attr_name, d, accessor=accessor)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/marshmallow/fields.py", line 252, in serialize
    return self._serialize(value, attr, obj)
  File "~/dev/restless_project/venv/lib/python2.7/site-packages/marshmallow_enum/__init__.py", line 74, in _serialize
    return value.name
AttributeError: 'unicode' object has no attribute 'name'

[justanr]

I should be able to look at this tonight, what database are you using?

[kiptoomm]

OK great. I'm using MySQL with the pymysql driver. If necessary, I can send you the link to a test project where you can reproduce this error

[justanr]

That'd be fantastic

[kiptoomm]

@justanr sure thing: https://github.com/kiptoomm/flask_and_restless

You wanna check out the branch 'test_marshmallow'. The commit that introduces the issue is 5eef4ad6ee508416bc0a7621241fb0c404e13b9b

[justanr]

I was able to reproduce this issue, however I don't think the issue is with this library. I dumped the __dict__ attr of the entity being output by patch:

{'_sa_instance_state': <sqlalchemy.orm.state.InstanceState object at 0x7fba62269e10>,
 'created_at': datetime.datetime(2018, 1, 19, 1, 56, 3, 957979),
 'first_name': 'A',
 'gender': 'MALE',
 'id': 1,
 'last_name': 'B',
 'updated_at': datetime.datetime(2018, 1, 19, 2, 17, 15, 372796)}

Compare to the one generated by the GET:

{'_sa_instance_state': <sqlalchemy.orm.state.InstanceState object at 0x7fba61dd7668>,
 'created_at': datetime.datetime(2018, 1, 19, 1, 56, 3, 957979),
 'first_name': 'A',
 'gender': <Gender.MALE: 0>,
 'id': 1,
 'last_name': 'B',
 'updated_at': None}

Note that this has the actual enum value where as the result of the patch has the string representation.

Given that the GET and POST don't usually explode (I put a raise in there), I'm going to hazard a guess that the issue probably lies in the glue between flask_restless and SQLAlchemy or just in flask_restless, however I'm not familiar enough that project to point you at anything in particular. Hopefully this puts you on the right track.

[kiptoomm]

@justanr do you have a gist that I can look at where you printed the __dict__ attr of the entity for both GET and PATCH? I'm having trouble catching the exceptions when using flask-restless pre/post_processors

[justanr]

I simply tossed a raise Exception(entity.__dict__) in the serialize and deserialize methods of the schema.

[kiptoomm]

Thanks for sending me in the right direction, @justanr. I think I've found the issue and following up with the flask-restless community. Details here: jfinkels/flask-restless#679