1090 fix(m2m): Return empty list if there are no m2m entries

[nVitius]

I ran into an issue where running getm2m without any existing m2m entries will throw an error that I believe is unintended.

[sinisaos]

@nVitius Thank you for your PR. I think this error is intentional and tells us that the list of values ​​cannot be empty. I think you should rather handle ValueError in your code directly using try/except. Sorry if I'm wrong.

[nVitius]

Seems unintuitive to throw an error instead of returning an empty list...
Also, the error isn't very obvious, as it's coming from the is_in call within the get_m2m method.

To clarify, the error is indicating that is_in didn't receive any id's to check. Not that there are no entries.

[dantownsend]

I'm just looking at this now.

[dantownsend]

Yeah, I think this is a bug.

@sinisaos You make a good point - if someone calls is_in directly with no values, then I think raising an error is OK. But for this particular query, the user isn't calling is_in directly, so we should handle this edge case and return an empty list.

[dantownsend]

Resolves #1090

[sinisaos]

Seems unintuitive to throw an error instead of returning an empty list... Also, the error isn't very obvious, as it's coming from the is_in call within the get_m2m method.

To clarify, the error is indicating that is_in didn't receive any id's to check. Not that there are no entries.

@nVitius Sorry for my mistake. Thanks for the clarification.

[dantownsend]

@sinisaos No need to apologise

[dantownsend]

OK, I'm going to throw up a quick test, then will get this merged in. Thanks both 👍

[dantownsend]

@nVitius Thanks a lot for the fix - it will be in the next release 👍

[dantownsend]

The fix is in piccolo==1.19.1.