The option is grayed-out in the portal setup dialogue.
How do I do it.
You need to create a relationship to another instance of your base table - a self-join. That table will be available for the portal.
Every portal requires that you first use Manage Database to create a relationship between two table occurrences. That's what we call the boxes on your relationships graph. You select one for your layout in layout setup and one for your portal in portal setup. The relationship that you define controls what records in your portal before any portal filter comes into play.
But you can use the same table for portal and layout if you create two occurrences of that table and link them in a relationship. To create a second occurrence of a table, click to select it and then click the button with two plus signs to make a copy of it.
you can have a self-join relationship - meaning you must connect the table to the same table
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