How to use the otree.db.models._OtreeModelFieldMixin function in otree

class SmallIntegerField(
        _OtreeNumericFieldMixin, models.SmallIntegerField):
    pass


class TextField(_OtreeModelFieldMixin, models.TextField):
    auto_submit_default = ''


class TimeField(_OtreeModelFieldMixin, models.TimeField):
    pass


class URLField(_OtreeModelFieldMixin, models.URLField):
    pass


ForeignKey = models.ForeignKey
ManyToOneRel = related.ManyToOneRel
ManyToManyField = models.ManyToManyField
OneToOneField = models.OneToOneField

# aliases we might use in the future
StringField = CharField
LongStringField = TextField

class SlugField(_OtreeModelFieldMixin, models.SlugField):
    pass


class SmallIntegerField(
        _OtreeNumericFieldMixin, models.SmallIntegerField):
    pass


class TextField(_OtreeModelFieldMixin, models.TextField):
    auto_submit_default = ''


class TimeField(_OtreeModelFieldMixin, models.TimeField):
    pass


class URLField(_OtreeModelFieldMixin, models.URLField):
    pass


ForeignKey = models.ForeignKey
ManyToOneRel = related.ManyToOneRel
ManyToManyField = models.ManyToManyField
OneToOneField = models.OneToOneField

# aliases we might use in the future
StringField = CharField
LongStringField = TextField

return super(BooleanField, self).clean(value, model_instance)

    def formfield(self, *args, **kwargs):
        from otree import widgets

        is_checkbox_widget = isinstance(self.widget, widgets.CheckboxInput)
        if not self._blank_is_explicit and is_checkbox_widget:
            kwargs.setdefault('required', False)
        else:
            # this use the allow_blank for the form fields
            kwargs.setdefault('required', not self.allow_blank)

        return super(BooleanField, self).formfield(*args, **kwargs)


class AutoField(_OtreeModelFieldMixin, models.AutoField):
    pass


class BigIntegerField(
        _OtreeNumericFieldMixin, models.BigIntegerField):
    auto_submit_default = 0


class BinaryField(_OtreeModelFieldMixin, models.BinaryField):
    pass


# FIXME: CharField should never be nullable, otherwise there is ambiguity
# when a form field is left empty (whether it's null or empty string)
class CharField(_OtreeModelFieldMixin, models.CharField):
    def __init__(

# themselves. also, "default" could be misleading -- people could think
        # it's the default choice in the form
        kwargs.setdefault('default', kwargs.pop('initial'))

        # if default=None, Django will omit the blank choice from form widget
        # https://code.djangoproject.com/ticket/10792
        # that is contrary to the way oTree views blank/None values, so to
        # correct for this, we get rid of default=None args.
        # setting null=True already should make the field null
        if 'default' in kwargs and kwargs['default'] is None:
            kwargs.pop('default')

        super(_OtreeModelFieldMixin, self).__init__(**kwargs)


class _OtreeNumericFieldMixin(_OtreeModelFieldMixin):
    auto_submit_default = 0

class BaseCurrencyField(
    _OtreeNumericFieldMixin, models.DecimalField,
    metaclass=models.SubfieldBase):

    MONEY_CLASS = None # need to set in subclasses

    def __init__(self, **kwargs):
        # i think it's sufficient just to store a high number;
        # this needs to be higher than decimal_places
        decimal_places = self.MONEY_CLASS.get_num_decimal_places()
        # where does this come from?
        max_digits=12
        super().__init__(
            max_digits=max_digits, decimal_places=decimal_places, **kwargs)

kwargs.setdefault('null', True)

        super(CharField, self).__init__(
            choices=choices,
            widget=widget,
            initial=initial,
            label=label,
            doc=doc,
            max_length=max_length,
            blank=blank,
            **kwargs)

    auto_submit_default = ''


class CommaSeparatedIntegerField(_OtreeModelFieldMixin,
                                 models.CommaSeparatedIntegerField):
    pass


class DateField(_OtreeModelFieldMixin, models.DateField):
    pass


class DateTimeField(_OtreeModelFieldMixin, models.DateTimeField):
    pass


class DecimalField(
        _OtreeNumericFieldMixin,
        models.DecimalField):
    pass

blank=blank,
            **kwargs)

    auto_submit_default = ''


class CommaSeparatedIntegerField(_OtreeModelFieldMixin,
                                 models.CommaSeparatedIntegerField):
    pass


class DateField(_OtreeModelFieldMixin, models.DateField):
    pass


class DateTimeField(_OtreeModelFieldMixin, models.DateTimeField):
    pass


class DecimalField(
        _OtreeNumericFieldMixin,
        models.DecimalField):
    pass


class EmailField(_OtreeModelFieldMixin, models.EmailField):
    pass


class FileField(_OtreeModelFieldMixin, models.FileField):
    pass

# "initial" is an alias for default. in the context of oTree, 'initial'
        # is a more intuitive name. (since the user never instantiates objects
        # themselves. also, "default" could be misleading -- people could think
        # it's the default choice in the form
        kwargs.setdefault('default', kwargs.pop('initial'))

        # if default=None, Django will omit the blank choice from form widget
        # https://code.djangoproject.com/ticket/10792
        # that is contrary to the way oTree views blank/None values, so to
        # correct for this, we get rid of default=None args.
        # setting null=True already should make the field null
        if 'default' in kwargs and kwargs['default'] is None:
            kwargs.pop('default')

        super(_OtreeModelFieldMixin, self).__init__(**kwargs)

class DateField(_OtreeModelFieldMixin, models.DateField):
    pass


class DateTimeField(_OtreeModelFieldMixin, models.DateTimeField):
    pass


class DecimalField(
        _OtreeNumericFieldMixin,
        models.DecimalField):
    pass


class EmailField(_OtreeModelFieldMixin, models.EmailField):
    pass


class FileField(_OtreeModelFieldMixin, models.FileField):
    pass


class FilePathField(_OtreeModelFieldMixin, models.FilePathField):
    pass


class FloatField(
        _OtreeNumericFieldMixin,
        models.FloatField):
    pass

class PositiveSmallIntegerField(
        _OtreeNumericFieldMixin,
        models.PositiveSmallIntegerField):
    pass


class SlugField(_OtreeModelFieldMixin, models.SlugField):
    pass


class SmallIntegerField(
        _OtreeNumericFieldMixin, models.SmallIntegerField):
    pass


class TextField(_OtreeModelFieldMixin, models.TextField):
    auto_submit_default = ''


class TimeField(_OtreeModelFieldMixin, models.TimeField):
    pass


class URLField(_OtreeModelFieldMixin, models.URLField):
    pass


ForeignKey = models.ForeignKey
ManyToOneRel = related.ManyToOneRel
ManyToManyField = models.ManyToManyField
OneToOneField = models.OneToOneField

# this use the allow_blank for the form fields
            kwargs.setdefault('required', not self.allow_blank)

        return super(BooleanField, self).formfield(*args, **kwargs)


class AutoField(_OtreeModelFieldMixin, models.AutoField):
    pass


class BigIntegerField(
        _OtreeNumericFieldMixin, models.BigIntegerField):
    auto_submit_default = 0


class BinaryField(_OtreeModelFieldMixin, models.BinaryField):
    pass


# FIXME: CharField should never be nullable, otherwise there is ambiguity
# when a form field is left empty (whether it's null or empty string)
class CharField(_OtreeModelFieldMixin, models.CharField):
    def __init__(
            self,
            *,
            choices=None,
            widget=None,
            initial=None,
            label=None,
            doc='',
            # varchar max length doesn't affect performance or even storage
            # size; it's just for validation. so, to be easy to use,