t = db.updated;
t.drop()

o = { _id : Math.random() , 
      items:[null,null,null,null]
    };

t.insert( o );
assert.eq( o , t.findOne() , "A1" );

o.items[0] = {amount:9000,itemId:1};
t.update({},{$set:{"items.0":o.items[0]}});
assert.eq( o , t.findOne() , "A2" );

o.items[0].amount += 1000;
o.items[1] = {amount:1,itemId:2};
t.update({},{$inc:{"items.0.amount":1000},$set:{"items.1":o.items[1]}});
assert.eq( o , t.findOne() , "A3" );