I inherited a django project which has mostly 'unit' tests that flex the ORM and the db, so they are really integration tests and are painfully slow. There is some important logic that happens in the ORM layer and that needs to be tested. At some point I want to find the time to mock the database so that they can be faster, but in some cases I worry about missing important interactions. Domain is highly specialized so not very easy to just know how to untangle the mess.

> I worry about missing important interactions.

If you are concerned that the ORM won't behave as it claims to, you can write tests targeted at it directly. You can then run the same tests against your mock implementation to show that it conforms to the same contract.

But an ORM of any decent quality will already be well tested and shouldn't do unexpected things, so perhaps the worry is for not?