Will removing a column with a Rails migration remove indexes associated with the column

问题

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)

回答1:

No, unfortunately you have to remove the index manually from within your migration using the remove_index method.

回答2:

From Rails 4 upwards, the index removes automatically with the column removal.

回答3:

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"

回答4:

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.

回答5:

In Rails > 3.2.16, removing the column removes the index.

回答6:

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