Sorry, marked this as the solution - but there is a problem.  I'm trying to do this from the metadata object (artwork) page.  The code would need to be more like this: {% assign related_products = collections['all'].products | where: 'metafields.art_...

Our site is both an artist's portfolio/gallery and store.  Each piece artwork is stored as  a metadata object.  They may or may not have prints/products available.  Each print/product has a reference to the artwork (metadata object) that it is based ...