Setting Store field value from other field values at load time

[G-Force]

Hi all,

I seem to recall an example that I've seen where a store field value or a grid column value was rendered by adding/subtracting two other fields at load time. I have tried all variations of search terms that I can think of but none of them has led me in the right direction so could someone please help me out?
Something like....

var timeStore = new Ext.data.JsonStore({
        url: '/gettime',
        method: 'POST',
        fields: [
           {name: 'fromtime', type: 'date', dateFormat: 'H:i'},
           {name: 'totime', type: 'date', dateFormat: 'H:i'},
           {name: 'length', type: 'number', defaultValue:totime.replace(/:/,"")-fromtime.replace(/:/,"")}
        ]    
}); 

Thanks,

G-Force

[G-Force]

I got it working using the ColumnModel renderer like this:

            {header: 'Length',
                width:50,
                dataIndex:'length',
                renderer: function(value, metaData, record, rowIndex, colIndex, store) {
                    var mDiff =  (parseInt(record.get('totime').dateFormat('H'))*60+parseInt(record.get('totime').dateFormat('i'))-(parseInt(record.get('fromtime').dateFormat('H'))*60+parseInt(record.get('fromtime').dateFormat('i'));
                    var sign = (mDiff<0?'-':'');
                    mDiff = Math.abs(mDiff);
                    var hDiff = Math.floor(mDiff/60);
                    mDiff = mDiff % 60;
                    return sign+(hDiff<10?'0':'')+hDiff+':'+(mDiff<10?'0':'')+mDiff;
                }  
            } 

Probably basic stuff for most and I'm sure the time subtraction can be more elegant but this does the job.

Thanks,

G-Force