there seem to be two problems. to fix them:
1. annotate custom method as [Query]
[Query]
public IQueryable<CustomerPurchase>
GetCustomerPurchases()
2. annotate custom class (T) with [Key]
public class CustomerPurchase
{
public CustomerPurchase()
{
}
[Key]
public string Title { get; set; }
// more code
}
after rebuilding the solution, the error message disappeared and the grid populated with expected data.