A relationship joined by date fails on form initialization.

[hitbox]

The following fails using sqlalchemy.Date, works using a custom date field that accepts a single datetime.date object. Appears to stem from match_pk in utils.find_entity, assuming col.type.python_type takes one argument.
Any chance find_entity (here) could handle this without a custom type?
Sorry if this is too long, wasn't sure of a better way to explain. Comment out DateType = MyDateType to get TypeError exception.

#!/usr/bin/env python2
import datetime

import sqlalchemy as sa
from sqlalchemy import create_engine, types
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from wtforms_alchemy import ModelForm, ModelFormField, ModelFieldList

engine = create_engine('sqlite:///:memory:')
Base = declarative_base(engine)
Session = sessionmaker(bind=engine)
session = Session()

class MyDate(datetime.date):

    def __new__(cls, arg):
        if isinstance(arg, datetime.date):
            return arg
        return MyDate.__new__(cls, arg)

class MyDateType(types.TypeDecorator):
    impl = types.Date

    @property
    def python_type(self):
        return MyDate


# Gives the error
DateType = sa.Date

# No error
DateType = MyDateType

class Report(Base):
    __tablename__ = 'reports'

    date = sa.Column(DateType, primary_key=True)


class Section(Base):
    __tablename__ = 'subreports'

    date = sa.Column(DateType, sa.ForeignKey('reports.date'), primary_key=True)
    section = sa.Column(sa.Integer, primary_key=True)

    report = sa.orm.relationship('Report', backref='sections')


class SectionForm(ModelForm):
    class Meta:
        model = Section
        include_primary_keys = True
        include_foreign_keys = True


class ReportForm(ModelForm):
    class Meta:
        model = Report
        include_primary_keys = True
        include_foreign_keys = True

    sections = ModelFieldList(ModelFormField(SectionForm))


Base.metadata.create_all()
report = Report(date=datetime.date(2015,12,13),
                sections=[Section(section=1)])
session.add(report)
session.commit()

form = ReportForm(obj=report)