I'm trying to work out how to generate totals of fields from related model records.

Example: I have 2 Google Drive Table models (Projects & Stages) with a relation where 1 Project can have many Stages. The Stages model has a number of fields, one of which is Budget.

I'm trying work out how to display a Total Project Budget, which is the sum of the Budget fields of the Stages related to each Project.

I'm really stuck on getting this to work and wondering if I need to create a client script, server script, or calculated model, or if there's an easy way to do this directly within a binding.

Here is a link to a bunch of screen-grabs of what I'm trying to build, the UI and setup in App Maker:

<https://www.dropbox.com/sh/wt00virvn88puyi/AACy7-fPLYFF1H9uFkvEuvgTa?dl=0>