Check whether there is no match in any row in SQL - sql

Question

schedule = Session.select()
.join(Activity).join(Course)
.join(StuCouRel)
.join(Student)
.where(Student.id == current_user.id)
.join(StuActIgnore).where(Activity.id != StuActIgnore.activity)
Now, if StuActIgnore.activity has, say, three rows in this query, then it will have each session that 'gets through' output three times. It will compare each Session to every row in StuActIgnore.activity. I know what's going wrong; I just don't know how to fix it.
I want something like this:
schedule = Session.select()
.join(Activity).join(Course).join(StuCouRel).join(Student)
.where(Student.id == current_user.id)
.join(StuActIgnore)
.where(Activity.id not in StuActIgnore.activity)
that is; if Activity.id does not match for any StuActIgnore row, then the Session is added to the query. The code I wrote does not work because a ForeignKeyField is not iterable.
EDIT: I tried fixing it as follows:
schedule = Session.select()
.join(Activity)
.join(Course)
.join(StuCouRel)
.join(Student)
.where(Student.id == current_user.id)
.join(StuActIgnore)
.where(Activity not in current_user.ignoredactivities)
because this related_name ("ignoredactivities") is iterable, but this doesn't seem to work either.
EDIT2: I made it work by adding a separate for-loop. It works, but it's neither efficient nor elegant:
ignored = Activity.select().join(StuActIgnore).where(StuActIgnore.student == current_user.id)
for element in schedule:
if element.activity not in ignored:
classes.append(element)
Is there a more elegant way of doing the same thing?