OrientDB is the most versatile DBMS supporting Graph, Document, Reactive, Full-Text and Geospatial models in one Multi-Model product. OrientDB can run distributed (Multi-Master), supports SQL, ACID Transactions, Full-Text indexing and Reactive Queries.
Changes in the TX commit protocol that will allow the added validation to enforce different types of n..n relations without the usage of indexes at all.
Currently, implementation of such methods requires knowledge of the internals of OrientDB and so should be provided by API.