product.metafields.custom["size"] =>
product.metafields.custom.size => 12

product.metafields.custom["first"] =>
product.metafields.custom.first => book_list["gid://shopify/Metaobject/592248854","gid://shopify/Metaobject/592216086","gid://shopify/Metaobject/660504598","gid://shopify/Metaobject/660439062"]

product.metafields.custom["last"] =>
product.metafields.custom.last =>

product.metafields.custom["to_h"] =>
product.metafields.custom.to_h =>

product.metafields.custom["to_liquid"] =>
product.metafields.custom.to_liquid =>