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?