Return any additional field options that need to be included as a
        result of uniqueness constraints on the model. This is returned as
        a two-tuple of:

        ('dict of updated extra kwargs', 'mapping of hidden fields')

def get_uniqueness_extra_kwargs(self, field_names, declared_fields, extra_kwargs):
        """
        Return any additional field options that need to be included as a
        result of uniqueness constraints on the model. This is returned as
        a two-tuple of:

        ('dict of updated extra kwargs', 'mapping of hidden fields')
        """
        model = getattr(self.Meta, 'model')
        model_fields = self._get_model_fields(
            field_names, declared_fields, extra_kwargs
        )

        # Determine if we need any additional `HiddenField` or extra keyword
        # arguments to deal with `unique_for` dates that are required to
        # be in the input data in order to validate it.
        unique_constraint_names = set()

        for model_field in model_fields.values():
            # Include each of the `unique_for_*` field names.
            unique_constraint_names |= {model_field.unique_for_date, model_field.unique_for_month,
                                        model_field.unique_for_year}

        unique_constraint_names -= {None}

        # Include each of the `unique_together` field names,
        # so long as all the field names are included on the serializer.
        for parent_class in [model] + list(model._meta.parents.keys()):
            for unique_together_list in parent_class._meta.unique_together:
                if set(field_names).issuperset(set(unique_together_list)):
                    unique_constraint_names |= set(unique_together_list)

        # Now we have all the field names that have uniqueness constraints
        # applied, we can add the extra 'required=...' or 'default=...'
        # arguments that are appropriate to these fields, or add a `HiddenField` for it.
        hidden_fields = {}
        uniqueness_extra_kwargs = {}

        for unique_constraint_name in unique_constraint_names:
            # Get the model field that is referred too.
            unique_constraint_field = model._meta.get_field(unique_constraint_name)

            if getattr(unique_constraint_field, 'auto_now_add', None):
                default = CreateOnlyDefault(timezone.now)
            elif getattr(unique_constraint_field, 'auto_now', None):
                default = timezone.now
            elif unique_constraint_field.has_default():
                default = unique_constraint_field.default
            else:
                default = empty

            if unique_constraint_name in model_fields:
                # The corresponding field is present in the serializer
                if default is empty:
                    uniqueness_extra_kwargs[unique_constraint_name] = {'required': True}
                else:
                    uniqueness_extra_kwargs[unique_constraint_name] = {'default': default}
            elif default is not empty:
                # The corresponding field is not present in the,
                # serializer. We have a default to use for it, so
                # add in a hidden field that populates it.
                hidden_fields[unique_constraint_name] = HiddenField(default=default)

        # Update `extra_kwargs` with any new options.
        for key, value in uniqueness_extra_kwargs.items():
            if key in extra_kwargs:
                extra_kwargs[key].update(value)
            else:
                extra_kwargs[key] = value

        return extra_kwargs, hidden_fields