Run callback when changing associated records

[adamnoto]

Sometimes, we want to wrap some logic around when an association is changed. I think this is a very valid use case.

Assume we have this class, where we want to automatically send an email when a division is updated, for review purpose.

class Workaty::Employee < Marten::Model
  field :id, :big_int, primary_key: true, auto: true
  field :team_divisions, :many_to_many, to: Workaty::TeamDivision, related: :members
end

If we do employee.team_divisions.add(team_division), the employee class has no idea that its association has changed thus we couldn't trigger the email sending, since after_update is not called. To send the review email, currently we define an add_team_division method, but this is definitely error-prone as we may forget its existence and call team_divisions.add instead.

This is a sketchy scenario indeed, we may argue "why not just send the email in the controller" or that it's best practice to do that instead of this. But yeah, I think it's such a common business case to wrap custom logic when relationship changes -- I can cite real business case example, but I am afraid I'd violate some agreements by doing so.

In Rails, they have something like touch. How about in Marten? Should we implement something like that, or do we already have a similar feature?

[ellmetha]

Hey @adamnoto! 👋

There is no such feature in Marten at the moment.

I will say that there are cons to implicit behaviours like the one you are suggesting (eg. what if you want to add a team division without triggering email sending? eg. while backfiling some data?). For these reasons, I would recommend defining your orchestration logic (adding records to associations and triggering emails) explicitly in a dedicated method or a dedicated abstraction (eg. service abstraction, operation, etc). Ideally you want to minimize the side effects induced by low level record operations (such as adding a record to a many-to-many association). Otherwise you are at risk of having these side effects being applied in situations in which you probably don't want to have them applied (e.g., backfilling, migrations, bulk imports).

Now I am not opposed to adding something like touch at some point in the future, but I would certainly not consider this a big priority at the moment. I will also review pull requests that would add this if this is something that you are interested in contributing to the framework.