If I’m reading your post, and the one you linked to, correctly, I think this can be easily done by expanding a record in your “master” table and then clicking on any linked records that you like. This can be done many layers deep, so you should be able to drill down as far as you’d like.

You can read more information in the documentation

And here are some example screenshots to help show this using one of the starter bases.

If this answers your question, please consider marking it as “solution”. If not, I’m happy to work with you further. Thanks!

But how can this be done in a form? (part of the OP question)

I’d like to send an order form to customers that they fill in. They’ll need to populate the form with their name from the customers table and the line items of the order form with products they select from the products table, add a quantity and have the form calculate the line total and the overall order total. The order data (date, customer #, order total) will be stored in the order table and the line item data (order #, product #, qty, line total) will be stored in the order items table. I’ve searched the help and forum and it appears that a form can only be used to populate fields in one table. Sorry if I missed it. Also, what’s the point of having a relational database if you don’t allow the end user to make use of the proper relations? Is there a way to send a more complex grid view to customers to have them fill in orders and line items?

We’ve built a solution for this exact problem. Our form lets you add line items from a different table and link them to the order.