need opinion from experts

I have three tables such as Plan, instalments, and collection. Plan has many instalments and instalment has one to many collections. On my view, I am showing plans and instalments. Plans are loaded in a combo, use can select plan from combo and load related instalments in a table below. I want to load sum of collection amounts, from multiple table rows, for each instalment and display this sum against instalment. I am not sure how to fetch it as part of instalment model in my controller and send it to view. Thanks in advance