arango> db.example.save({"a" : { "one" : 1, "two" : 2, "three" : 3 }, "b" : { }});
{ "error" : false, "_id" : "example/20774803", "_key" : "20774803", "_rev" : "20774803" }
arango> db.example.update("example/20774803", {"a" : { "four" : 4 }, "b" : { "b1" : 1 }});
{ "error" : false, "_id" : "example/20774803", "_key" : "20774803", "_rev" : "21168019" }
arango> db.example.document("example/20774803");
{ "a" : { "one" : 1, "two" : 2, "three" : 3, "four" : 4 }, "b" : { "b1" : 1 }, "_id" : "example/20774803", "_key" : "20774803", "_rev" : "21168019" }

arango> db.example.update("example/20774803", { "a" : { "one" : null }, "b" : null }, false, false);
{ "error" : false, "_id" : "example/20774803", "_key" : "20774803", "_rev" : "21364627" }
arango> db.example.document("example/20774803");
{ "a" : { "two" : 2, "three" : 3, "four" : 4 }, "_id" : "example/20774803", "_key" : "20774803", "_rev" : "21364627" }