arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a" ] }); { "deduplicate" : true, "fields" : [ "a" ], "id" : "test/15259", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : false, "code" : 201 } arangosh> db.test.save({ a : 1 }); { "_id" : "test/15262", "_key" : "15262", "_rev" : "_VRerBWK---" } arangosh> db.test.save({ a : 1 }); { "_id" : "test/15266", "_key" : "15266", "_rev" : "_VRerBWO---" } arangosh> db.test.save({ a : null }); { "_id" : "test/15269", "_key" : "15269", "_rev" : "_VRerBWO--_" }