The ForeignKey functionality provide you with the ability to display and edit a foreign key column with a human readable lookup collection instead of working with IDs.
The only requirement is to pass the collection with the Text/Value pairs at the time you define the Grid - it cannot be dynamically extracted from your model used for the Grid.
Please notice that your suggestion
columns.ForeignKey(s => s.FaceId, x => x.FacesList)
would mean that it is possible to have different collection for each record which is not true - same Text/Value pair collection is used for each of the records.
the Telerik team
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