Backend Query filter from dropdown document reference

I have 2 collections in the DB:

shows

classes

classes has a filed for show_ref which is the reference for a document in the shows collection.

In the interface, I want 2 dropdowns, one for show and one for class.

The shows dropdown lists all the items in the shows collection.

I want the classes dropdown to only have the classes with that show reference.

I have set up the shows dropdown to use the show_ref as the data, and the name as the label.

I have set up the classes dropdown with a query to the classes collection. I wanted to then filter to the widget state of the shows dropdown, however it shows as a Text element an in red so I cannot select it.

I have watched videos and read the docs but can't see a solution to filter using a document reference. the knowledgebase uses text elements.

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