class Repository(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4)

    prefix = models.ForeignKey(
        "RepositoryPrefix",
        related_name="repository_set",
        on_delete=CASCADE,
        blank=False,
        null=True
    )
    ...
    
    class Meta:
        unique_together = [("prefix", "name")]
        ...

class RepositorySerializer(serializers.ModelSerializer):
    prefix = serializers.SerializerMethodField()
    ...

    def get_prefix(self, obj):
        return obj.prefix.name if obj.prefix else None