Eclipse Hawk is a heterogeneous model indexing framework: it indexes collections of models transparently and incrementally into a NoSQL database, which can be queried in a more efficient and convenient manner. You can mirror EMF, UML or Modelio models (among others) into a Neo4j or OrientDB graph, which you can query with their native languages, or (preferably) through the languages provided by Hawk. Hawk will watch over those models and update the graph whenever they change, in an incremental manner.
Current versions of Hawk integrate extended versions of the Epsilon Object Language and the Epsilon Pattern Language (part of the Eclipse Epsilon project). The advantage of using these languages is that you can reuse the same query across backends - it will work the same across Neo4j, OrientDB or Greycat.
Hawk also includes tools to make life easier with it: exposing Hawk queries as EMF models, or a web service API for querying remote Hawk indexes over the network.