With Search API + Facet API it is possible to create search facets for taxonomy term reference fields, that reflect the hierarchy of terms in the vocabulary, based on the taxonomy_term_parent table. (With Facet API Taxonomy Folding it is even possible to browse these hierarchies as folders.)
Sometimes hierarchical information is not represented in the taxonomy_term_parent table, and the relations exist between terms of different vocabularies.
E.g. in a site I am building, a "Product series" has a term reference field to "Product family", which has a term reference to a "Supplier". Suppliers, Product families and Product series are 3 distinct taxonomy vocabularies.
Some products only have a supplier. Others have a supplier and a product family. And some have all three. If a product has a series, then its product family is the same as that of the series. And if it has a product family, then the supplier is the same as that of the product family.
I would like to expose this structure as one hierarchical facet, similar in appearance and behavior to that for a single hierarchical vocabulary.
What I managed so far is to assemble the terms from different vocabularies into a common entity property. I also managed to unify the two different "parent term" relationships (series -> family, family -> supplier) into a common entity property. But then what?