Unique Boolean Condition Constraint

I'd like to add a unique constraint (via add_index or something similar) in a migration which works off two columns, one of which is boolean and must be true for the constraint to hold. Something like..

         add_index :my_table, [:some_integer_column, {:some_boolean_column => true}], :unique => true

..but of course this doesn't work. Is there a "correct" way of doing this without writing the raw SQL?