Make operation with aggregate function result

Question

I confess that I do not get along very well with the Deferred object. I'm making a query to the database on several "Stores" and as a result I want to do a series of operations. This troubles me because the results are returned asynchronously and I have no way to perform the corresponding operation on the "store" you should. In short, the problem is that this piece of code always executes the same function on the same "Store"

for (var i = 0; i < schema['stores'].length; i++) {
        storeName = schema['stores'][i].name;

        var objeto = db.executeSql('SELECT MAX(date_upd) FROM ' + '"' + storeName + '"').done(
                function(result, a){
                    //saveDataSynce(db, storeName, result);
                    console.log(result);
                }
        );
}

Answer 1

Whenever there is a loop on async operation, be very careful about function scope. In your example code, storeName inside the function will always be the last executed value. Use function scope as follow:

var getMax = function(storeName) {
  db.executeSql('SELECT MAX(date_upd) FROM ' + '"' + storeName + '"').done(
    function(result){
       //saveDataSynce(db, storeName, result);
       console.log(storeName, result);
    }
  );
}

for (var i = 0; i < schema['stores'].length; i++) {
    getMax(schema['stores'][i].name);
}

However, preferred coding pattern for YDN-DB is NoSQL style as follow:

var getMax = function(storeName) {
  var indexName = 'date_upd';
  var key_range = null; // whole store
  var limit = 1;
  var offset = 0;
  var reverse = true;
  db.values(storeName, indexName, key_range, limit, offset, reverse).done(
    function(results) {
       var max_key = results[0]; // may be undefined. OK.
       //saveDataSynce(db, storeName, max_key);
       console.log(storeName, max_key);
    }
  );
}

Note that keys (primary or index) are always sorted by ascending order. Max key is the first key in reverse order.