arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a", "b.c" ], unique: true }); { "deduplicate" : true, "fields" : [ "a", "b.c" ], "id" : "test/108612", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : true, "code" : 201 } arangosh> db.test.save({ a : 1, b : { c : 1 } }); { "_id" : "test/108615", "_key" : "108615", "_rev" : "_XPb92de--_" } arangosh> db.test.save({ a : 1, b : { c : 1 } }); [ArangoError 1210: unique constraint violated - in index 108612 of type hash over ["a","b.c"]; conflicting key: 108615] arangosh> db.test.save({ a : 1, b : { c : null } }); { "_id" : "test/108621", "_key" : "108621", "_rev" : "_XPb92di--_" } arangosh> db.test.save({ a : 1 }); [ArangoError 1210: unique constraint violated - in index 108612 of type hash over ["a","b.c"]; conflicting key: 108621]