diff --git a/jdav_web/members/models.py b/jdav_web/members/models.py
index eec124d..dd617f2 100644
--- a/jdav_web/members/models.py
+++ b/jdav_web/members/models.py
@@ -1278,14 +1278,14 @@ class Freizeit(CommonModel):
             return sum([i.duration for i in self.ljpproposal.intervention_set.all()])
         else:
             return 0
-    
+
     @property
     def total_seminar_days(self):
         """calculate seminar days based on intervention hours in every day"""
+        # TODO: add tests for this
         if hasattr(self, 'ljpproposal'):
             hours_per_day = (
-                Intervention.objects
-                .filter(ljp_proposal_id=self.ljpproposal.id)
+                self.ljpproposal.intervention_set
                 .annotate(day=TruncDate('date_start'))  # Extract the date (without time)
                 .values('day')  # Group by day
                 .annotate(total_duration=Sum('duration'))  # Sum durations for each day
@@ -1302,17 +1302,17 @@ class Freizeit(CommonModel):
     def ljp_duration(self):
         """calculate the duration in days for the LJP"""
         return min(self.duration, self.total_seminar_days)
-   
+
     @property
     def staff_count(self):
         return self.jugendleiter.count()
-    
+
     @property
     def staff_on_memberlist(self):
         ps = set(map(lambda x: x.member, self.membersonlist.distinct()))
         jls = set(self.jugendleiter.distinct())
         return ps.intersection(jls)
-    
+
     @property
     def staff_on_memberlist_count(self):
         return len(self.staff_on_memberlist)
@@ -1322,7 +1322,7 @@ class Freizeit(CommonModel):
         ps = set(map(lambda x: x.member, self.membersonlist.distinct()))
         jls = set(self.jugendleiter.distinct())
         return len(ps - jls)
-    
+
     @property
     def head_count(self):
         return self.staff_on_memberlist_count + self.participant_count