Transform to a MongoDB-safe value.

def to_foreign(self, obj, name, value):  # pylint:disable=unused-argument
		"""Transform to a MongoDB-safe value."""
		
		from marrow.mongo import Document
		
		kind = self._kind(obj if isclass(obj) else obj.__class__)
		
		if isinstance(value, Document):
			if __debug__ and kind and issubclass(kind, Document) and not isinstance(value, kind):
				raise ValueError("Not an instance of " + kind.__name__ + " or a sub-class: " + repr(value))
			
			return value
		
		if isinstance(kind, Field):
			kind.validator.validate(value, FieldContext(kind, obj))
			return kind.transformer.foreign(value, FieldContext(kind, obj))
		
		if kind:
			value = kind(**value)
		
		return value