STI - Polymorphism

Hi everyone,

I'm having some trouble with my models, here's the case.

I have an Activity table, wich has many Task ( another table), and
then I have a table called Assignment.
Between Assignment and Activity there's a many-to-many relation, so I
have another table AssignmentActivity

When I want to enter data I start by filling activities, after that I
create a group of tasks for that activity. Lets say that I have create
5 activities and each one has 7 tasks.

Now I create an assignment, and after that I create the activities for
this assignment. Let's say that I only choose 3 of the 5 activities
and for those 3 activites I only choose 2 tasks of the 7 I had. If
thats the case I've made my tables this way

Activity Task

Assignment AssignmentActivity

But I realized that the difference between ASSIGNMENTACTIVITY and
ACTIVITY table is only 3 columns, and it happens the same between TASK
and ASSIGNMENTACTIVITYTASK, and I'm not sure if I could use single
table inheritance in this case, or maybe I'm doing it in a wrong way.

Is my approach ok? Hope someone can help me with this

Thanks in advance

Javier Q.