How do I add associations to the model?

I have a Transaction and TransactionId which is associated with each Transaction. The TransactionId is a composite key, so, I wrote a new property and have overridden the convert function. When I use alert to get the properties, it is returning undefined.

If I put the following code, I am not able to retrieve the properties inside the TransactionId model.

Code:

Ext.define('Transaction', {
        extend: 'Ext.data.Model',
        idProperty: 'assetTxId',
        fields: [{
            name: 'aset01fkCurrCd'
        }, {
            name: 'firmStatus'
        }, {
            name: 'amt',
            type: 'float'
        }, {
            name: 'valueDate',
            type: 'date',
            dateFormat: 'Y-m-d'
        }, {
            name: 'assetTransType'
        }, {
            name: 'assetTxId', 
            type:'string',
            convert: function(value,record) {
                alert(record.get('creationDate'));
                return record.get('creationDate') 
                        + record.get('nbr') 
                        + record.get('dcoId');
            }
        }],
        hasOne: [{
            name: 'TransactionId',
            model: 'TransactionId',
            associationKey: 'id' // <- this is the same as what is in the JSON response
        }],


    });


    Ext.define('TransactionId', {
        extend: 'Ext.data.Model',
        fields: [{
                name: 'creationDate',
                type: 'date',
                dateFormat: 'Y-m-d'
        }, {
            name: 'nbr',
            type: 'float'
        }, {
            name: 'dcoId',
        }]
    });

However, If I put all the properties in the same model, I am able to retrieve the properties when I alert. Could you suggest if I am doing anything wrong?

Here is my working code.

Code:

  Ext.define('Transaction', {
        extend: 'Ext.data.Model',
        idProperty: 'TransactionId',
        fields: [{
            name: 'aset01fkCurrCd'
        }, {
         name: 'firmStatus'
        }, {
            name: 'amt',
            type: 'float'
        }, {
            name: 'valueDate',
            type: 'date',
            dateFormat: 'Y-m-d'
        }, {
         name: 'assetTransType'
        }, {
         name: 'creationDate',
         mapping: 'id.creationDate',
         type: 'date',
            dateFormat: 'Y-m-d'
        }, {
         name: 'nbr',
         mapping: 'id.nbr',
         type: 'numberfield'
        }, {
         name: 'dcoId',
         mapping: 'id.dcoId',


        }, {
         name: 'TransactionId', 
         type:'string',
         convert: function(value,record) {
             return record.get('creationDate') 
                     + record.get('nbr') 
                     + record.get('dcoId');
         }
        }]
    });

The json data is as follows:

Code:

[
    {
        "id": {
            "creationDate": 1393567200000,
            "nbr": 619,
            "dcoId": "CDO"
        },
        "amt": 65000000,
        "aset01fkCurrCd": "USD",
        "assetTransType": "DW",
        "firmStatus": "VRFYD",
        "valueDate": 1393567200000
    },
    {
        "id": {
            "creationDate": 1393567200000,
            "nbr": 513,
            "dcoId": "CDO"
        },
        "amt": -4607329.03,
        "aset01fkCurrCd": "USD",
        "assetTransType": "WD",
        "firmStatus": "VRFYD",
        "valueDate": 1393567200000
    },
    {
        "id": {
            "creationDate": 1393567200000,
            "nbr": 598,
            "dcoId": "CDO"
        },
        "amt": 69305.02,
        "aset01fkCurrCd": "USD",
        "assetTransType": "DW",
        "firmStatus": "VRFYD",
        "valueDate": 1393567200000
    }
]