Hi Uri, That was the approach taken by an older problem called django-evolution; the main problem with that approach is that it means the migrations that happen are different on every system depending on the database, down to the point where a system with frequent schema updates might detect a rename whereas one that gets the schema lumped on it in one go might diff incorrectly (as it's not seen the intermediary stages) and start deleting and making models when it shouldn't.
In addition, there's no place to put things like data migrations. The second option (which was taken by the old dmigrations project) was project-wide migrations rather than per-app, allowing circular dependencies to all be done in a single migration, much like syncdb did them all in a single pass. The problem here, of course, is that this makes it impossible to ship migrations with third-party apps, so they don't have any way of doing more complex changes than adding/removing a field because of the "dumb diff" issues - it's impossible to always pick a field/model rename versus a model addition and deletion. So, we have the app-based migration current system, which allows people to run the diff (that's what the Autodetector and makemigrations does), and then review and refine its output first before preserving the final, polished migrations on disk in a distributable format that follows Django's app-based structure. I hope that helps. Andrew On Thu, Oct 16, 2014 at 8:16 AM, Joe Tennies <[email protected]> wrote: > First this is really a question for the django-users group. This is more > for actually developing Django. I think the root of your issue is actually > the app loading feature and not migrations. Your models should be lazy > loading each other (using the string version not directly importing). > > Secondly, the diff approach is difficult to get right, especially when you > consider data migration as well as schema migration. You can have a tested > path from a to d via b and c. It's tough to know how to get from a to d > without b and c. And the permutations grow exponentially. The Python > Community and especially the Django people are heavily into testing being > an important (read required) part of development and change integration > testing. > On Oct 16, 2014 5:38 AM, "Uri P" <[email protected]> wrote: > >> I have to admit I don't know the ins and outs of the new migration system >> in django1.7+, and I don't quite understand why it fails to handle circular >> dependencies which the old fashioned syncdb does. I would love to hear an >> explanation, but my main question is: >> >> I*f the initial syncdb is actually capable of creating the database for >> the models despite of the circular dependencies, why not base the migrating >> mechanism simply on some kind of DIFF between the existing database and the >> one that would have been created by syncdb for the newly changed models?* >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Django developers (Contributions to Django itself)" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To post to this group, send email to [email protected]. >> Visit this group at http://groups.google.com/group/django-developers. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/django-developers/cef5b6e4-9f2e-4c81-aaee-0eede10d55b6%40googlegroups.com >> <https://groups.google.com/d/msgid/django-developers/cef5b6e4-9f2e-4c81-aaee-0eede10d55b6%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> For more options, visit https://groups.google.com/d/optout. >> > -- > You received this message because you are subscribed to the Google Groups > "Django developers (Contributions to Django itself)" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > Visit this group at http://groups.google.com/group/django-developers. > To view this discussion on the web visit > https://groups.google.com/d/msgid/django-developers/CACiOJ6s4APj4fWcugVA3Bn2_Z1NvtE64dWg65Xoch5vb8xSQ8w%40mail.gmail.com > <https://groups.google.com/d/msgid/django-developers/CACiOJ6s4APj4fWcugVA3Bn2_Z1NvtE64dWg65Xoch5vb8xSQ8w%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/django-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/CAFwN1upyXX_zmquV-YtT9ef4NjP%3DR2CSr99pONZjBzKaFgMO8g%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
