Hi there, @svenschuldt… if I understand your post correctly (and using a recent post from you that I happened to remember to get a bit more context), I think you should be able to get a count of the to do’s like this… Search for DB_todos:count with a constraint on the search of milestone_id = Search for DB_milestones:first item with a constraint on the search of 1_project_id = Parent group's DB_project.
If you wanted to simplify things a bit, you could have a field on the DB_todo data type that stores the project with which a to do is associated. Also, just a heads up… the id text fields you are showing are not creating the links between the data types. Those links exist because you have tied the fields in DB_milestone and DB_todo to the actual data types, and therefore, the id text fields probably aren’t necessary.
Hope this helps.
Best…
Mike