On your Invoice DataGrid you add an Edit Button to open a page with a single Invoice.
On the Invoice Page you add a DataGrid with DataSource = Database over Association.
This is not an answer, but a comment with an image:
So although it appears that I have the page set up appropriately (see below), while the data from the invoice (parent) displays correctly, the invoice_line (child) data never appears. Am I missing something?
I am coming to believe my problem is due to a fundamental misunderstanding about how Mendix handles data. I imported the data for my two entities from excel spreadsheets. Each entity has an invoice_number. But defining an association between entities in Mendix doesn't take into account shared attributes. Is there any way to modify or expose this behavior?
Since my goal is to develop an application that can operate in offline mode, using XPath or microflows to connect a parent with its children is forbidden. Given my constraints, is there any way to make this work? Or should I be looking at non-Mendix solutions?