Fun story: I once worked on a C piece of industrial software that was running on AIX 3.x to 6.x; the goal was to have a nicer (web) status monitoring UI as well as being able to implement stuff in a "safer" language and onboard more people.

The main problem was that the "database" was IBM C-ISAM. Think MySQL MyISAM tables, except don't even dream about SQL, you interact directly with the internal primitives through a C API; when you'r used to SQL it feels like bitbanging in ASM.

The plan:

- Write a Python binding to the C-ISAM library

- Write a subset of the Django ORM from scratch that would use the above behind the scenes; of course it's more limited but whatever's there must behave the same.

- Write any new software using that subset; slowly port over the old code to the new software; of course it can't use _everything_ that one would otherwise use in a normal app; but then again C-ISAM was so constrained that expectations were incredibly limited anyway from the very beginning.

- [much later] pivot! swap out the mock-Django models and drop in the real Django ORM (basically s/from mockdjango import/from django import/g) and hit some mysql/psql/whathaveyou that you've populated with the C-ISAM schema and data

- All the software written is all the merrier and Just Works; the world's your oyster.

This was made possible because the Django ORM is _incredibly simple_: the PoC was done in an afternoon, the hardest part being understanding Python meta classes.