arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a", "b.c" ], unique: true }); { "deduplicate" : true, "fields" : [ "a", "b.c" ], "id" : "test/111498", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : true, "code" : 201 } arangosh> db.test.save({ a : 1, b : { c : 1 } }); { "_id" : "test/111501", "_key" : "111501", "_rev" : "_Y2g68ze--_" } arangosh> db.test.save({ a : 1, b : { c : 1 } }); [ArangoError 1210: unique constraint violated - in index 111498 of type hash over ["a","b.c"]; conflicting key: 111501] arangosh> db.test.save({ a : 1, b : { c : null } }); { "_id" : "test/111507", "_key" : "111507", "_rev" : "_Y2g68zi--B" } arangosh> db.test.save({ a : 1 }); [ArangoError 1210: unique constraint violated - in index 111498 of type hash over ["a","b.c"]; conflicting key: 111507]