arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a", "b.c" ], unique: true }); { "deduplicate" : true, "fields" : [ "a", "b.c" ], "id" : "test/16170", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : true, "code" : 201 } arangosh> db.test.save({ a : 1, b : { c : 1 } }); { "_id" : "test/16173", "_key" : "16173", "_rev" : "_WpTBAZy--_" } arangosh> db.test.save({ a : 1, b : { c : 1 } }); [ArangoError 1210: unique constraint violated - in index 16170 of type hash over ["a","b.c"]; conflicting key: 16173] arangosh> db.test.save({ a : 1, b : { c : null } }); { "_id" : "test/16179", "_key" : "16179", "_rev" : "_WpTBAZy--D" } arangosh> db.test.save({ a : 1 }); [ArangoError 1210: unique constraint violated - in index 16170 of type hash over ["a","b.c"]; conflicting key: 16179]