How to implement a unique index on two columns in rails

问题

I have a table and I'm trying to add a unique index on two columns. Those columns are also indexed. So my question is if I just can remove the indexes who were just for one column or if I have to use all three indexes:

add_index "subscriptions", ["user_id"]
add_index "subscriptions", ["content_id"]
add_index "subscriptions", ["user_id"], ["content_id"], :unique => true

回答1:

add_index :subscriptions, [:user_id, :content_id], unique: true

来源：https://stackoverflow.com/questions/4123610/how-to-implement-a-unique-index-on-two-columns-in-rails