Querying history on a model instance¶
HistoricalRecords object on a model instance can be used in the same
way as a model manager:
>>> from polls.models import Poll, Choice >>> from datetime import datetime >>> poll = Poll.objects.create(question="what's up?", pub_date=datetime.now()) >>> >>> poll.history.all() [<HistoricalPoll: Poll object as of 2010-10-25 18:03:29.855689>]
Whenever a model instance is saved a new historical record is created:
>>> poll.pub_date = datetime(2007, 4, 1, 0, 0) >>> poll.save() >>> poll.history.all() [<HistoricalPoll: Poll object as of 2010-10-25 18:04:13.814128>, <HistoricalPoll: Poll object as of 2010-10-25 18:03:29.855689>]
Querying history on a model class¶
Historical records for all instances of a model can be queried by using the
HistoricalRecords manager on the model class. For example historical
records for all
Choice instances can be queried by using the manager on the
Choice model class:
>>> choice1 = poll.choice_set.create(choice_text='Not Much', votes=0) >>> choice2 = poll.choice_set.create(choice_text='The sky', votes=0) >>> >>> Choice.history <simple_history.manager.HistoryManager object at 0x1cc4290> >>> Choice.history.all() [<HistoricalChoice: Choice object as of 2010-10-25 18:05:12.183340>, <HistoricalChoice: Choice object as of 2010-10-25 18:04:59.047351>]
Because the history is model, you can also filter it like regularly QuerySets,
Choice.history.filter(choice_text='Not Much') will work!
Getting previous and next historical record¶
If you have a historical record for an instance and would like to retrieve the previous historical record (older) or next historical record (newer), prev_record and next_record read-only attributes can be used, respectively.
>>> from polls.models import Poll, Choice >>> from datetime import datetime >>> poll = Poll.objects.create(question="what's up?", pub_date=datetime.now()) >>> >>> record = poll.history.first() >>> record.prev_record None >>> record.next_record None >>> poll.question = "what is up?" >>> poll.save() >>> record.next_record <HistoricalPoll: Poll object as of 2010-10-25 18:04:13.814128>
If a historical record is the first record, prev_record will be None. Similarly, if it is the latest record, next_record will be None
Reverting the Model¶
SimpleHistoryAdmin allows users to revert back to an old version of the
model through the admin interface. You can also do this programmatically. To
do so, you can take any historical object, and save the associated instance.
For example, if we want to access the earliest
HistoricalPoll, for an
Poll, we can do:
>>> poll.history.earliest() <HistoricalPoll: Poll object as of 2010-10-25 18:04:13.814128>
And to revert to that
HistoricalPoll instance, we can do:
>>> earliest_poll = poll.history.earliest() >>> earliest_poll.instance.save()
This will change the
poll instance to have the data from the
HistoricalPoll object and it will create a new row in the
HistoricalPoll table indicating that a new change has been made.
The HistoryManager allows you to query a point in time for the latest historical
records or instances. When called on an instance’s history manager, the
method will return the instance from the specified point in time, if the instance
existed at that time, or raise DoesNotExist. When called on a model’s history
as_of method will return instances from a specific date and time
that you specify, returning a queryset that you can use to further filter the result.
>>> t0 = datetime.now() >>> document1 = RankedDocument.objects.create(rank=42) >>> document2 = RankedDocument.objects.create(rank=84) >>> t1 = datetime.now() >>> RankedDocument.history.as_of(t1) <HistoricalQuerySet [ <RankedDocument: RankedDocument object (1)>, <RankedDocument: RankedDocument object (2)> ]> >>> RankedDocument.history.as_of(t1).filter(rank__lte=50) <HistoricalQuerySet [ <RankedDocument: RankedDocument object (1)> ]>
as_of is a convenience: the following two queries are identical.
If you filter by pk the behavior depends on whether the queryset is returning instances or historical records. When the queryset is returning instances, pk is mapped to the original model’s primary key field. When the queryset is returning historical records, pk refers to the history_id primary key.
is_historic and to_historic¶
If you use as_of to query history, the resulting instance will have an attribute named _history added to it. This property will contain the historical model record that the instance was derived from. Calling is_historic is an easy way to check if an instance was derived from a historic point in time (even if it is the most recent version).
You can use to_historic to return the historical model that was used to furnish the instance at hand, if it is actually historic.
If you have two historic tables linked by foreign key, you can change it to use a HistoricForeignKey so that chasing relations from an as_of acquired instance (at a specific point in time) will honor that point in time when accessing the related object(s). This works for both forward and reverse relationships.
See the HistoricForeignKeyTest code and models for an example.
This method will return the most recent copy of the model available in the model history.
>>> from datetime import datetime >>> poll.history.most_recent() <Poll: Poll object as of 2010-10-25 18:04:13.814128>
Save without a historical record¶
If you want to save a model without a historical record, you can use the following:
class Poll(models.Model): question = models.CharField(max_length=200) history = HistoricalRecords() def save_without_historical_record(self, *args, **kwargs): self.skip_history_when_saving = True try: ret = self.save(*args, **kwargs) finally: del self.skip_history_when_saving return ret poll = Poll(question='something') poll.save_without_historical_record()
Or disable history records for all models by putting following lines in your
SIMPLE_HISTORY_ENABLED = False
Filtering data using a relationship to the model¶
To filter changes to the data, a relationship to the history can be established. For example, all data records in which a particular user was involved.
class Poll(models.Model): question = models.CharField(max_length=200) log = HistoricalRecords(related_name='history') Poll.objects.filter(history__history_user=4)
You can also prefetch the objects with this relationship using something like this for example to prefetch order by history_date descending:
Poll.objects.filter(something).prefetch_related(Prefetch('history', queryset=Poll.history.order_by('-history_date'), to_attr='ordered_histories')