HistoricOneToOneField

[valberg]

Problem Statement
We have just gotten HistoricForeignKey which is great! It would be nice to have a similar for OneToOneField.

Describe the solution you'd like
Implementation of HistoricOneToOneField which does the same as HistoricForeignKey just within the bounds of a OneToOneField.

[quadracik]

Hi, here's my implementation, basing on HistoricForeignKey. I'm still testing it, but feel free to take a look.

class HistoricForwardOneToOneDescriptor(ForwardOneToOneDescriptor):
    """
    Overrides get_queryset to provide historic query support, should the
    instance be historic (and therefore was generated by a timepoint query)
    and the other side of the relation also uses a history manager.
    """

    def get_queryset(self, **hints) -> QuerySet:
        instance = hints.get("instance")
        if instance:
            history = getattr(instance, SIMPLE_HISTORY_REVERSE_ATTR_NAME, None)
            histmgr = getattr(
                self.field.remote_field.model,
                getattr(
                    self.field.remote_field.model._meta,
                    "simple_history_manager_attribute",
                    "_notthere",
                ),
                None,
            )
            if history and histmgr:
                return histmgr.as_of(getattr(history, "_as_of", history.history_date))
        return super().get_queryset(**hints)


class HistoricReverseOneToOneDescriptor(ReverseOneToOneDescriptor):
    """
    Overrides get_queryset to provide historic query support, should the
    instance be historic (and therefore was generated by a timepoint query)
    and the other side of the relation also uses a history manager.
    """

    def get_queryset(self, **hints):
        instance = hints.get("instance")
        if instance:
            try:
                return instance._prefetched_objects_cache[
                    self.related.field.remote_field.get_cache_name()
                ]
            except (AttributeError, KeyError):
                history = getattr(
                    instance, SIMPLE_HISTORY_REVERSE_ATTR_NAME, None
                )
                histmgr = getattr(
                    self.related.model,
                    getattr(
                        self.related.model._meta,
                        "simple_history_manager_attribute",
                        "_notthere",
                    ),
                    None,
                )
                if history and histmgr:
                    queryset = histmgr.as_of(
                        getattr(history, "_as_of", history.history_date)
                    )
        return super().get_queryset(**hints)


class HistoricOneToOneField(models.OneToOneField):
    """
    Allows one to one foreign keys to work properly from a historic
    instance.

    If you use as_of queries to extract historical instances from
    a model, and you have other models that are related by one to
    one foreign key and also historic, changing them to
    a HistoricOneToOneField field type will allow you to naturally
    cross the relationship boundary at the same point in time
    as the origin instance.

    A historic instance maintains an attribute ("_historic") when
    it is historic, holding the historic record instance and the
    timepoint used to query it ("_as_of").  HistoricOneToOneField
    looks for this and uses an as_of query against the related
    object so the relationship is assessed at the same timepoint.
    """

    forward_related_accessor_class = HistoricForwardOneToOneDescriptor
    related_accessor_class = HistoricReverseOneToOneDescriptor

[JordanHyatt]

Sorry I didn't see this when I opened issue #1389

[quadracik]

No problem. We've been using it in production for over a year and I didn't experience any problems with it. Unfortunately I don't have enought free time to write propper tests and open a good pull request.

[JordanHyatt]

So I am trying to write the pull request now. The HistoricForwardOneToOneDescriptor you wrote seems to be working fine. However, I think you have a bug in your HistoricReverseOneToOneDescriptor class.

if history and histmgr:
      queryset = histmgr.as_of(
          getattr(history, "_as_of", history.history_date)
      )               

I think you should be returning here and not setting it to the variable "queryset". So I think you are always just getting the non-historical queryset because the super is always called.

when I return instead, there are errors. So I am working on those now.

[JordanHyatt]

Found it....you I think you needed self.related.related_model instead of self.related.model

[quadracik]

@JordanHyatt , actually now you mentioned it, I checked and the final solution we use also returns the queryset instead assigning it and uses self.related.related_model.
There is one more difference - the final code doesn't do try .. except. Instead, it only does what is in the except clause. I don't remember why, but it must have been causing some problems.. Or maybe it only applies to reverse FKs as they may have prefetch_related used on them and for 1to1 it never goes this way. Not sure now. But please have it in mind.
My final code for HistoricReverseOneToOneDescriptor below:

class HistoricReverseOneToOneDescriptor(ReverseOneToOneDescriptor):
    """
    Overrides get_queryset to provide historic query support, should the
    instance be historic (and therefore was generated by a timepoint query)
    and the other side of the relation also uses a history manager.
    """

    def get_queryset(self, **hints):
        instance = hints.get("instance")
        if instance:
            history = getattr(
                instance, SIMPLE_HISTORY_REVERSE_ATTR_NAME, None
            )
            histmgr = getattr(
                self.related.related_model,
                getattr(
                    self.related.related_model._meta,
                    "simple_history_manager_attribute",
                    "_notthere",
                ),
                None,
            )
            if history and histmgr:
                return histmgr.as_of(
                    getattr(history, "_as_of", history.history_date)
                )
        return super().get_queryset(**hints)

[JordanHyatt]

That's funny, all 3 of those things I noticed and adjusted. See the PR I submitted. Thanks for posting, it definitely gave me a good headstart

[quadracik]

Great, haven't noticed it. Thanks for your work!