hi I'm trying to use convert config of field
but get this error
TypeError: 'undefined' is not an object (evaluating 'record.retailer.name')

this is my model

Code:
function retailerDetail(v, record){
    return record.retailer.name + ', ' + record.retailer.retailerID+','+record.retailer.URL;
}
function locationDetail(v,record)
{
    return record.location.line1+','+record.location.line2+','+record.location.city+','+record.location.postcode;
    
}
function vatDetail(v,record)
{
    return record.vat.rate+','+record.vat.net+','+record.vat.vat+','+record.vat.gross;
}
Ext.regModel('receipts_model', {
    fields:[
    {name:'timestamp',type:'auto'},
    {name:'receiptID',type:'auto'},
    {name:'Identifier',type:'auto'},
    {name:'retailer',convert:retailerDetail},
    {name:'name',mapping:'retailer.name'},
    {name:'retailerID',mapping:'retailer.retailerID'},
    {name:'URL',mapping:'retailer.URL'},
    {name:'transactionID',type:'auto'},
    {name:'postcode',type:'auto'},
    {name:'location',convert:locationDetail},
    {name:'line1',mapping:'location.line1'},
    {name:'line2',mappimg:'location.line2'},
    {name:'city',mapping:'location.city'},
    {name:'locationPostcode',mapping:'location.postcode'},
    {name:'total' ,type:'auto'},
    {name:'vat',convert:vatDetail},
    {name:'rate',mapping:'vat.rate'},
    {name:'net',mapping:'vat.net'},
    {name:'vatAmount',mapping:'vat.vat'},
    {name:'gross',mapping:'vat.gross'},

    ],
    associations:[
                       {type: 'hasMany', model: 'testItemModel'},
                    {type:'hasMany',model:'testOfferModel'}
                 ],
 
 });
this is my store
Code:
LoginForm.receipts_store =new Ext.data.Store({
    model  : 'receipts_model',
    sorters: 'purchaseDate',
  
  data: [
        {timestamp: '15:32 15 October 2011', receiptID: '123451', Identifier:'9876543210',retailer:{name:'Marks and Spencers',retailerID:'770000000001',URL:'images/Morrisons Logo.jpg'},transactionID:'5487854241',postcode:"EC3V3NP",total:'52.78',vat:{rate:'20',net:'43.98',vat:'8.8',gross:'52.78'}},
       
        ] });
the data for which i'm trying to create the model
Code:
[     {         "timestamp":"15:32 15 October 2011",         "receiptID":123451,         "identifier":9876543210,         "retailer":{             "name":"Marks and Spencers",             "retailerID":"770000000001",             "retailer_image_url":"http://paprless.com/770000000001/public.png"         },         "transactionID":5487854241,         "postcode":"EC3V3NP",         "location":{             "line1":"25 West 2nd Street",             "line2":"",             "city":"London",             "postcode":"EC3V3NP"         },         "item":[             {"name":"SCIENCE PUTTY",                 "qty":1,                 "amount":5},             {"name":"WNC STRAW MLW LPOP",                 "qty":1,                 "amount":0.79},             {"name":"RIDLEYS CLASSIC JOKE",                 "qty":1,                 "amount":7},             {"name":"BRUT CHAMPAGNE MAG",                 "qty":1,                 "amount":39.99}         ],         "total":52.78,         "vat":{             "rate":20,             "net":43.98,             "vat":8.8,             "gross":52.78},         "returns":[         ],         "offer":{             "offerID":555555,             "condition":"Buy One",             "reward":"Get One Free",             "creator":"Travis Perkins",             "status":"live",             "image_url":"http://paprless.com/offers/555555/smart.png"         }     }  ]
i'm also not sure whether i'm using the convert config properly
also i want to know how to map the values returned by the function

please guide