arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a", "b.c" ], unique: true }); { "deduplicate" : true, "fields" : [ "a", "b.c" ], "id" : "test/16303", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : true, "code" : 201 } arangosh> db.test.save({ a : 1, b : { c : 1 } }); { "_id" : "test/16306", "_key" : "16306", "_rev" : "_WtcBTbq--_" } arangosh> db.test.save({ a : 1, b : { c : 1 } }); [ArangoError 1210: unique constraint violated - in index 16303 of type hash over ["a","b.c"]; conflicting key: 16306] arangosh> db.test.save({ a : 1, b : { c : null } }); { "_id" : "test/16312", "_key" : "16312", "_rev" : "_WtcBTbu--B" } arangosh> db.test.save({ a : 1 }); [ArangoError 1210: unique constraint violated - in index 16303 of type hash over ["a","b.c"]; conflicting key: 16312]