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Model field Definitions
There is no way to add the 'convert' property to a field definition.
Essentially to create a virtual or calculated field the convert property is required, this is not available and cannot be added via the Component Config 'Add' function as this only add string values not function references or closures.
Sencha - Architect Dev Team
In the next update, you will be able to add a convert function to Ext.data.Field via a configuration.
Sencha Architect Development Team