@mgazdzik It's definitely a bug. I've assigned myself, since I changed the purger in another PR. We can perhaps get the column names by looking for 'idaction' columns.

It could be a way, but it will not allow any custom plugins to prevent their data in log_action from being deleted, and I imagine that it's gonna be tricky to enforce all custom plugins to use "idaction_" prefix for their dictionary columns?
My personal oppinion is to go for something easily extendible, but core issue will be solved by either of approaches.

It could be a way, but it will not allow any custom plugins to prevent their data in log_action from being deleted,

I see your point. We could use DI, alllow plugins to append column names to the array. This would also avoid creating a new event for something this small.

If it's documented well and easy to extend for developers, then I'm 💯 % for it :)

I'm not 100% into this topic. Ideally everything would just work automatically as know matter what a developer has to do, it will be forgotten etc. A custom plugin developer won't be able to know whether to listen to an event or DI or ... Is it maybe possible to use somehow existing information for this? Eg we have those Dimension classes: https://github.com/piwik/piwik/blob/2.14.0-b1/plugins/Contents/Columns/ContentName.php and know whether it defines an idAction and we also have those Action extensions https://github.com/piwik/piwik/blob/2.14.0-b1/plugins/Contents/Actions/ActionContent.php

If not possible to use any of this information, can we maybe refactor Dimensions in a way that it would be possible? If not, ignore this comment :) As I said I'm not 100% into this topic

I thought about that, it would be better if it was automatic. The only thing I could think of is to store a property in Dimension like $isActionReference. Problems w/ this approach include:

1. It exposes an implementation detail in the 'concept' layer (so-to-speak). In my mind, Dimension classes shouldn't be tied to relational DB implementation details (such as column type, or whether the actual dimension data is stored in a different table (ie, log_action)). Since we already do this, and Piwik's not likely to support NoSQL anytime soon, I guess it's not that important.
2. Plugin developers would still have to be aware of this and set it.
3. If a plugin developer doesn't use Dimension classes, there's no way to add the column.

Could do both, add a property to DImension classes & allow manipulation via DI. Then if a plugin developer finds out there's a problem w/ their plugin + log data purging, they can at least fix it.

1. We can absolutely ignore NoSQL. Once this will be implemented we'll have to refactor things anyway. We don't need to think about that for now. It's rather important to make things for developers as nice and simple as possible.
2. That sucks a bit. I was hoping we could use that https://github.com/piwik/piwik/blob/2.14.0-b1/plugins/Contents/Actions/ActionContent.php#L39 and that https://github.com/piwik/piwik/blob/2.14.0-b1/plugins/Contents/Columns/ContentName.php#L38 information. The tracker does already create a reference based on that automatically. I'm really not into this topic that's why it might sound stupid: Isn't it possible to check whether the ContentName defines a getIdAction method and a onLookupAction? The logic in Action is currently here: https://github.com/piwik/piwik/blob/2.14.0-b1/core/Tracker/Action.php#L295
3. We don't want to allow new Dimensions in the future that are not defined why a Dimension class.

In general I do not like having two solutions for one problem. It makes it always complicated and one never knows which one to use etc. Still hoping to fix it automatically to avoid plugin developers to notice that there's a log purger problem :)

Is it required for dimensions that are log_action references to implement getActionId()? If so, then I can use this.

Yep, if they want to use an action reference. It throws an exception otherwise: https://github.com/piwik/piwik/blob/2.14.0-b1/core/Plugin/Dimension/ActionDimension.php#L173

Will make use of it then.

@tsteur getActionId() is only for ActionDimensions. Is it possible for developers to create VisitDimensions & ConversionDimensions that are action references?

I'm not aware on any re VisitDimension or log_visit. A ConversionDimension neither from what I know.

Fixed by #7964