t = db.set3;
t.drop();

t.insert( { "test1" : { "test2" : { "abcdefghijklmnopqrstu" : {"id":1} } } } );
t.update( {}, {"$set":{"test1.test2.abcdefghijklmnopqrstuvwxyz":{"id":2}}})

x = t.findOne();
assert.eq( 1 , x.test1.test2.abcdefghijklmnopqrstu.id , "A" );
assert.eq( 2 , x.test1.test2.abcdefghijklmnopqrstuvwxyz.id , "B" );