#1270982 by drunken monkey, dereine: Allow to specify entity type on non-base-tables, which allows some fancy views integrations like a generic entity views integration

Issue #470258 by joachim after more than a year of work: Groupwise representative relationships. Or subquery relationships.

Issue #1188500: Provide a reverse relationship on term reference fields and a generic entity reverse relationship handler so anyone can use it.

Use the term_tid filter handler on field term filters, modified slightly to restrict vocabulary correctly.