When implementing related drop downs in a batch editing grid one should populate the secondary combo using client-side binding. This is needed because if a postback occurs while the user edits the data its changes will be lost. That said you can populate the related combo by calling a web service or executing a callback.
In attachments you can find a sample website that follows the second suggestion and populates the RadComboBox
by subscribing to its OnItemsRequested
event. Note that in the concrete example uses dummy data but the logic can be modified in such a manner so that a SQL database is used.
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