add collection method

        ids.append(str(self.pk))

        return ":".join(ids)

    def to_path(self):

        titles = []

        for ancestor in self.ancestors():

            titles.append(ancestor.title)

        titles.append(self.title)

        return titles

class MetadataCollectionValue(models.Model):

    metadata = models.ForeignKey(Metadata, on_delete=models.CASCADE)