arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a", "b.c" ], unique: true }); { "deduplicate" : true, "fields" : [ "a", "b.c" ], "id" : "test/16324", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : true, "code" : 201 } arangosh> db.test.save({ a : 1, b : { c : 1 } }); { "_id" : "test/16327", "_key" : "16327", "_rev" : "_XG3zIfS--_" } arangosh> db.test.save({ a : 1, b : { c : 1 } }); [ArangoError 1210: unique constraint violated - in index 16324 of type hash over ["a","b.c"]; conflicting key: 16327] arangosh> db.test.save({ a : 1, b : { c : null } }); { "_id" : "test/16333", "_key" : "16333", "_rev" : "_XG3zIfW--_" } arangosh> db.test.save({ a : 1 }); [ArangoError 1210: unique constraint violated - in index 16324 of type hash over ["a","b.c"]; conflicting key: 16333]