How do I handle too long index names in a Ruby on Rails ActiveRecord migration?

I am trying to add an unique index that gets created from the foreign keys of four associated tables:

add_index :studies,
  ["user_id", "university_id", "subject_name_id", "subject_type_id"],
  :unique => true

The database’s limitation for the index name causes the migration to fail. Here’s the error message:

Index name ‘index_studies_on_user_id_and_university_id_and_subject_name_id_and_subject_type_id’ on table ‘studies’ is too long; the limit is 64 characters

How can I handle this? Can I specify a different index name?