apps.get_model in RunPython returning "LookupError: No installed app with label"

[Gordon Wrigley]

This is just a possible space for catching user errors and warning about them.
I have two apps, a backfill migration created with --empty and using RunPython in app A is using apps.get_model to get a model from app B.
However the migration has no dependency on app B.
Depending on runtime ordering of migrations this may cause a LookupError, or it may work.
It feels like apps.get_model could be noticing that based on the dependency graph the referenced app/model may not exist.

[Adam Johnson]

apps.get_model() only runs when the migration runs. Are you proposing that when it runs, it infers that the current migration is reachable through *some* migration ordering without app B's model being available, even though in the *current* migration ordering, things are fine? That seems fairly complicated.

[Gordon Wrigley]

Pretty much yes.
I don't know how it works internally, I appreciate the complexity will depend a lot on how much access it has to the migration DAG when the individual migrations run.

[Carlton Gibson]

Hey Gordon. Thanks for this.

I'm going to say wontfix as it strikes me as not-practical — maybe it's doable but at what cost vs Add the dependency by hand when writing the migration, which seems pretty fair (at least at first pass.)

Having said that, if you want to provide a proof-of-concept we can have a look at it to make a more informed decision.

I hope that makes sense.

[Gordon Wrigley]

I agree the resolution is easy once you know what the problem is.
The difficult bit is working out what caused the error, especially when it occurs a month after the missing dependency because some later migration caused the ordering to change.
It's rather baffling to end up staring at an error that says your app isn't installed when it manifestly is, and it was working yesterday and is working in prod.

[Carlton Gibson]

Sure, and (as I said) happy to look at a suggestion here. My reservation is that I'm sceptical it would be worth the complexity. (If you're writing a RunPython that uses app B, it's clear — I think — that you should add the dependency.)

[Gordon Wrigley]

I don't know if it makes it any easier, but the narrower feature of requiring a dependency on an app A migration in order to use apps.get_model(A, ...) would probably cover it, it forces you to stop and think about the dependencies