Hi Philipp,
You should use the Database option. If you notice the entity path, it is over an association. So the way it works is that it will retrieve all the Registration records from database which are associated with the parent TrainingEvent object.
You should do like this
Hi all, the image which course asks appears when you use a Connector tab in your Studio Pro (it is located close to the Properties and Toolbox tab). Select a list view, then you'll see in the Connector available associations and entities. Click on the required entity and drag and drop it over the list view, then you'll see the image you shared.
The other ways to set the data source, suggested in answers, are also correct. The course however lets you experience the ease of use of Connector tab ;-)
Did exactly that and do not get this window. Either one or the other, not both options: