ExtJS Model fields have mapping option.

```
fields: [
        {name: 'brandId', mapping:'brand.id',   type: 'int'},
        {name: 'brandName', mapping:'brand.name', type: 'string'},

```

The problem is: if the response from server does not contain some field(brand field in my example) and mapping from inner fields is defined, Ext Store silently fails to load any records.

Does anybody have problems with this? Is it some kind of a bug?

**UPDATE**
To make it clear: suppose I have ten fields in my model. Response from server has nine fields, one is missing. If there is no nested mapping for this field (mapping:'x.y.z') everything is OK - store loads record, the field is empty. But if this field has to be loaded from some nested field and has mapping option - store fails to load ANYTHING.

**UPDATE 2**
I have found the code, that causes problems. The fact is: when Ext tries to load some field from Json it performs a check like this 

```
(source["id"] === undefined) ? __field0.defaultValue : source["id"]

```

But when field has mapping option(mapping 'brand.id') Reader does it this way

```
(source.brand.id === undefined) ? __field20.defaultValue : source.brand.id

```

which causes error if source has no brand field.

In case you have same problems as I: you can fix it by overloading *Ext.data.reader.Json*'s method *createFieldAccessExpression*