Sorry for the late answer.
Generally when you need to edit such properties (that represent an object (not some primitive value)) no matter if this is a navigation property or not the recommended way to do this is via GridViewComboBoxColumn. However GridViewComboBoxColumn has some known issues when too many items are set as ComboBox.ItemsSource (since this will create as many comboboxes as many visible rows you have on the screen), so very simple workaround is to set CellTemplate to something lightweight visual object (such as TextBlock) and bind it to same value as GridViewComboBoxColumn.DisplayMemberPath. At the end we have a fast grid in view mode and the power of ComboBox editing in edit mode.
You can change the editing value within RadGridView.CellEditEnded event. You can determine if this is a commit or cancel operation by checking GridViewCellEditEndedEventArgs.EditAction property.
Let me know if there is something unclear or you need further assistance.
the Telerik team
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