In previous versions one could add e.g. a filter field for their index to
a relationship, such as 'Categories.ID' where categories is a many_many
relationship. This stopped working in the SS4 upgrade. Namespaces, whoop.
Knowing if a DataObject has a table or not is irrelevant - data is queried
out and pushed into solr all the same, whether there is a join table, a
base table, or any combination with maybe a few no-tables in between.
SomeDataClass::get() will fetch all data irrelevant of tables, that's the
point of an abstraction like SilverStripe's ORM.
