/**
 * Computes a sum of data in the model.
 **/
Db.CalcSum = new Class
({
	Extends: Db.Calc
	,Tag: 'db-calc-sum'

	,getRowValue: function (row)
	{
		var value;
	
		if (this._func)
		{
			this.form.row = row;
			value = this._func (this.form);
		}
		else
			value = this._model.getByIndex (row, this.columnIndex);

		return value;
	}

	,before: function (row)
	{
		var value = this.getRowValue (row)
	
		if (value !== null)
		{
			this.sum -= value;
			this.numbersCount--;
		}
	}

	,after: function (row)
	{
		var value = this.getRowValue (row);

		if (value !== null)
		{
			this.sum += value;
			this.numbersCount++;
		}
	}

	,init: function ()
	{
		this.sum = 0;
		this.numbersCount = 0;
	}

	,done: function ()
	{
		if (this.numbersCount > 0)
			this.value = this.sum;
		else
			this.value = null;
	}
});