Error when a model contains a column_property

from marshmallow_sqlalchemy import ModelSchema
from sqlalchemy import *
from sqlalchemy.ext.declarative import declared_attr, declarative_base
from sqlalchemy.orm import *

Base = declarative_base()


class Foo(Base):
    __tablename__ = 'foo'
    id = Column(Integer, primary_key=True)

    @declared_attr
    def moo(self):
        return column_property(select([1337]))


e = create_engine('sqlite:///')
Base.metadata.create_all(e)
s = Session(e)

s.add(Foo())
s.commit()

foo = s.query(Foo).first()
print foo.moo


class FooSchema(ModelSchema):
    class Meta:
        model = Foo

Traceback

Traceback (most recent call last):
  File "satest.py", line 29, in <module>
    class FooSchema(ModelSchema):
  File "/home/adrian/dev/indico/env/lib/python2.7/site-packages/marshmallow/schema.py", line 116, in __new__
    dict_cls=dict_cls
  File "/home/adrian/dev/indico/env/lib/python2.7/site-packages/marshmallow_sqlalchemy/schema.py", line 60, in get_declared_fields
    declared_fields = mcs.get_fields(converter, opts, base_fields, dict_cls)
  File "/home/adrian/dev/indico/env/lib/python2.7/site-packages/marshmallow_sqlalchemy/schema.py", line 94, in get_fields
    dict_cls=dict_cls,
  File "/home/adrian/dev/indico/env/lib/python2.7/site-packages/marshmallow_sqlalchemy/convert.py", line 105, in fields_for_model
    field = base_fields.get(prop.key) or self.property2field(prop)
  File "/home/adrian/dev/indico/env/lib/python2.7/site-packages/marshmallow_sqlalchemy/convert.py", line 125, in property2field
    field_class = field_class or self._get_field_class_for_property(prop)
  File "/home/adrian/dev/indico/env/lib/python2.7/site-packages/marshmallow_sqlalchemy/convert.py", line 186, in _get_field_class_for_property
    field_cls = self._get_field_class_for_column(column)
  File "/home/adrian/dev/indico/env/lib/python2.7/site-packages/marshmallow_sqlalchemy/convert.py", line 153, in _get_field_class_for_column
    return self._get_field_class_for_data_type(column.type)
  File "/home/adrian/dev/indico/env/lib/python2.7/site-packages/marshmallow_sqlalchemy/convert.py", line 178, in _get_field_class_for_data_type
    'Could not find field column of type {0}.'.format(types[0]))
marshmallow_sqlalchemy.exceptions.ModelConversionError: Could not find field column of type <class 'sqlalchemy.sql.sqltypes.NullType'>.

[sloria]

This is a known issue. I’m not quite sure how to solve this since column_properties can return any type. For now, use an explicit field

    total_tasks = fields.Integer(dump_only=True)

[jebrage]

This works for me.

class FooSchema(ModelSchema):
    moo = ma_fields.Integer(dump_only=True)

    class Meta:
        model = Foo

        fields = (
          Foo.moo.key,
        )

        dump_only = (
            Foo.moo.key,
        )

This seems right as we should only need the property during serialization and dumping. We shouldn’t need it to deserialize and load.