Orchard Migration Add_Index to an existing table t

问题:

Looking at Q A migration to add unique constraint to a combination of columns

In my migration file I already have a 'MobileDeviceRecord' table with an 'ActivationCode' which is created in code from a GUID class when a new record is created. I want to ensure the new 'ActivationCode' is unique to the existing 'ActivationCodes' in the table. Do I need to remove the 'ActivationCode' column and re-add it with a constraint or can I use an add_index...Also what is the proper way to use it??

public int UpdateFrom82()
    {
        add_index :MobileDeviceRecord, [:ActivationCode], :unique => true
        return 83;
    }