Wrapping persistencejs methods to use jQuery Promises

persistence-promises-example.js

//This depends on jquery and persistencejs.

/**
 * Given a"model" Product. Instances of Product persist their data to Web SQL (persistencejs)
 */
var Product = persistence.define('products', {
	name: 'TEXT',
	image_path: 'TEXT',
	created_at: 'DATE',
	updated_at: 'DATE'
});

/**
 * If we want info from db, we do something like.
 */
persistence.transaction(
  function(tx){
      tx.executeSql(
          "select max(updated_at) as max_updated_at from products", 
          null,
          function(row){
          	//this is a callback used to receive the results from db
          }
      );
  }
);

/**
 * Then we might want to encapsulate that logic into a method of our Product model.
 */ 
Product.getMaxUpdatedAt = function(callback){
  var deferred = new $.Deferred(); 
  persistence.transaction(
    function(tx){
        tx.executeSql(
            "select max(updated_at) as max_updated_at from products", 
            null,
            callback
        );
    }
  );
};

/**
 * And use the method by creating a callback to get the query result
 */
var aCallback = function(row){console.log(row);}
Product.getMaxUpdatedAt(callback);

/**
 * But lets suppose we need to getMaxUpdatedAt() of several models synchrounously and do something only 
 * when all of them are fetched from db. We would end up writing something like
 */
var maxs = [];
Product.getMaxUpdatedAt(function(productMax){
  maxs.push(productMax);
  Category.getMaxUpdatedAt(function(categoryMax){
    maxs.push(categoryMax);
    User.getMaxUpdatedAt(function(userMax){
      maxs.push(userMax);
      //Notice how we can get deeper and deeper
      maxs.sort(function(a,b){return a-b})
    })
  })
})

/**
 * To avoid deeply nested callbacks we can use Promises.
 * We can refactor the getMaxUpdatedAt() methods to use jQuery.Deferred() to return a Promise, just like $.ajax does.
 */ 
Product.getMaxUpdatedAt = function(){
	var deferred = new $.Deferred(); 
	persistence.transaction(
    function(tx){
        tx.executeSql(
            "select max(updated_at) as max_updated_at from products", 
            null,
            function(row){
            	deferred.resolve(row[0]['max_updated_at']);
            }
        );
    }
);
	return deferred.promise();
};

/**
 * So if we want to sort all the maxs ...
 */
var maxResults = [];
maxResults.push(Category.getMaxUpdatedAt());
maxResults.push(Product.getMaxUpdatedAt());
maxResults.push(User.getMaxUpdatedAt());

$.when.apply($, maxResults).then(function(maxCategory, maxProduct, maxUser){
  maxs = [maxCategory, maxProduct, maxUser].sort(function(a, b){return a-b})
});