In order to achieve the desired functionality you should manually updated your data source with the value selected from the RadComboBox. First you should subscribe on the SelectedIndexChanged event of the RadComboBox. In the event handler function you should get reference to the GridDataItem which is currently being edited. This could be done by accessing the NamingContainer of the RadComboBox. Once you have a reference to the GridDataItem you will find the corresponding record in the database and append the newly selected value to the existing one, so when the update command is executed the Grid will be bound to this new value.
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