In Rails 2, will removing a column with a Rails migration also change/remove indexes associated with the column? If not, and instead you have to also change/remove each index manually, shouldn't it instead be automated?
Thanks (from a Rails newbie)
No, unfortunately you have to remove the index manually from within your migration using the remove_index method.
From Rails 4 upwards, the index removes automatically with the column removal.
To clarify, inside a migration the syntax to remove a 2 column index is the following
remove_index :actions, :column => [:user_id,:action_name]
or by name, a worse option from my point of view
remove_index :actions, :name => "index_actions_on_user_id_and_action_name"
Just as a caution, while Rails 4 will remove the index for you if you remove the column, you should specify the column type. Without a column type, running rake db:rollback will return
rake aborted!
StandardError: An error has occurred, all later migrations canceled:
remove_column is only reversible if given a type.
I was experimenting with dropping foreign key columns that were indexed. Even specifying index: true in the change block didn't seem to make the columns reversible on rollback.
In Rails > 3.2.16, removing the column removes the index.
If you want to remove index you should use remove_index, if you use remove_column it does remove the index but you can't run rake db:rollback. As Jim mentioned.
remove_column is only reversible if given a type.
来源：https://stackoverflow.com/questions/7204476/will-removing-a-column-with-a-rails-migration-remove-indexes-associated-with-the