arango> db.test.ensureHashIndex("a");
{ "id" : "test/5922391", "unique" : false, "type" : "hash", "fields" : ["a"], "isNewlyCreated" : true }

arango> db.test.save({ a : 1 });
{ "_id" : "test/6381143", "_key" : "6381143", "_rev" : "6381143" }

arango> db.test.save({ a : 1 });
{ "_id" : "test/6446679", "_key" : "6446679", "_rev" : "6446679" }

arango> db.test.save({ a : null });
{ "_id" : "test/6708823", "_key" : "6708823", "_rev" : "6708823" }