I'm new to AppGini and do not get ahead ...

AppGini Version 5.84
Structure, simplified:
table 'logging' - fields: id, duration, project [lookup, id], activity [lookup, id]
table 'projects' - fields: id, name
table 'activities' - fields: id, activity
table (relation) 'r_prj_act' - fields: id, project_id, activity_id => (each project has a set of activities)
table (relation) 'r_ma_prj' - fields: id, ma ( = <%%creatorUsername%%>), prj => (each user has a set of projects)
Result - in each case one direct and one indirect lookup works:
Case 1:
table 'logging' - project [lookup to table 'projects'], activity [lookup to table 'r_prj_act']
I get all projects and the desired set of activities.
Case 2:
table 'logging' - project [lookup to table 'r_ma_prj'], activity [lookup to table 'activities']
With modified ajax_combo (WHERE `r_ma_prj`.`ma`="'.$_SESSION['memberID']) I get the user specific set of projects and all activities.
But I need a double indirect lookup:
table 'logging' - project [lookup to table 'r_ma_prj'], activity [lookup to table 'r_prj_act']
Desired result: user specific set of projects and the project specific set of activities.
I can not get these two things combined. Thanks a lot for help!
Markus