Because sequence number was running number per "tenant" (in my case it was invoice type) that started from zero (0) at the beginning of the every year.
You could use max+1 as well but you should be aware that there is a slim chance that there is race condition and you may end up having two entities with the same max number - that though would raise error when trying to save into database. By having explicitly pessimistic locking (select_for_update) for a row with tenant + sequence value you can guarantee that no other request can change the value while you're reading it. To make that easier you can create custom manager that can retrieve updated sequence values automagically, so your call would look like something like: Tenant.sequence.get_value(tenant, sequence) On Tue, 13 Jan 2015 06:37:21 -0800 (PST) Matt Cooper <vtbassm...@gmail.com> wrote: > Thanks Jani, good to hear someone has gone down this road before. > Since you did it in code, is there a reason you kept a sequence value > on Tenant rather than using > Widget.objects.aggregate(Max(Widget.sequence_value)) ? I ask because > for my app, Widget is actually just one of several cases where I need > this pattern. If I could keep all the book-keeping on the model with > the constraint, that seems cleaner to me. But, of course, I'm not > willing to pay a huge performance or corruption penalty just for > vague ideological purity :) > > Thanks again! > ~matt > > On Tuesday, January 13, 2015 at 5:07:21 AM UTC-8, Jani Tiainen wrote: > > > > I've done something similiar. > > > > Only way to do that is that you need to upkeep that tenant > > sequence_id manually, > > > > So doing something like: > > > > class Tenant(models.Model): > > name = models.CharField(max_length=50) > > sequence_value = models.IntegerField() > > > > and in a code: > > > > tenant = Tenant.object.get(id=tenant_id).select_for_update() # > > Pessimistic locking > > tenant.sequence_value = tenant.sequence_value + 1 > > tenant.save() > > > > widget = Widget(owner=tenant,sequence_value=tenant.sequence_value) > > widget.save() > > > > That should do the trick. Another option could be using table > > triggers to automate > > that within a database table trigger(s). > > > > On Mon, 12 Jan 2015 21:26:54 -0800 (PST) > > Matt Cooper <vtbas...@gmail.com <javascript:>> wrote: > > > > > I'm building a multi-tenant application with a data model that > > simplifies > > > to: > > > > > > class Tenant(models.Model): > > > name = models.CharField(max_length=50) > > > > > > class Widget(models.Model): > > > owner = models.ForeignKey(Tenant) > > > sequence_id = <question here> > > > > > > > > > I want Widget.sequence_id to work like an AutoField (that is, > > automatically > > > give me something unique and generally incrementing by 1) but > > > scoped to > > a > > > single Widget.owner. So for instance, when Tenant A creates 3 > > > widgets, > > they > > > would get sequence_ids 1, 2, and 3. Then Tenant B comes along and > > creates 2 > > > widgets; those widgets get sequence_ids 1 and 2. The tenant/user > > > gets no control over the number, but it's something she'll see in > > > the UI. AutoNumber is out because from the perspective of a > > > single tenant, they should have sequential widget IDs. > > > GUIDs/UUIDs are out because it needs > > to > > > be human-readable. > > > > > > I looked through docs on AutoField and unique_together. I turned > > > StackOverflow upside down and only found this > > > <http://stackoverflow.com/questions/18072586/django-autofield-increment>, > > > > > which isn't quite what I want. I didn't see anything on > > djangopackages.com > > > that would solve my problem, though I could have missed it. > > > > > > Basically, has anyone done something like this before and have a > > suggested > > > pattern? > > > > > > Thanks! > > > ~matt > > > > > > -- > > > You received this message because you are subscribed to the > > > Google > > Groups "Django users" group. > > > To unsubscribe from this group and stop receiving emails from it, > > > send > > an email to django-users...@googlegroups.com <javascript:>. > > > To post to this group, send email to django...@googlegroups.com > > <javascript:>. > > > Visit this group at http://groups.google.com/group/django-users. > > > To view this discussion on the web visit > > https://groups.google.com/d/msgid/django-users/37fd32e7-28dd-4c10-8451-b0d705a3e52e%40googlegroups.com. > > > > > > > For more options, visit https://groups.google.com/d/optout. > > > > -- > You received this message because you are subscribed to the Google > Groups "Django users" group. 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