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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)

Dexygen
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  • By the way here's an interesting post that does not necessarily directly answer my question however: http://keyj.wordpress.com/2009/05/28/remove-index-with-rails-migrations/ – Dexygen Aug 26 '11 at 12:28
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    Update: currently, it does remove the index (Rails 4.1.7). – B Seven Nov 29 '14 at 16:38

6 Answers6

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

x6iae
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enter08
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    Although removing column will remove its index, it won't be fully reversible. Rollback will not add the index back. In this case `remove_index` is needed – Peter T. May 24 '19 at 09:02
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No, unfortunately you have to remove the index manually from within your migration using the remove_index method.

John Topley
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    for MySQL 5.1 (at least) this does not appear to be the case. See: http://stackoverflow.com/a/4341928/ – Tyler May 10 '13 at 23:33
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    In Rails 4 it does now: http://stackoverflow.com/a/27622694/407213 (e.g.: I tried and after `remove_column ...`, `remove_index ...` throws a "Index name '...' on table '...' does not exist" <3 ) – Dorian Jun 16 '16 at 17:50
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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"
Maragues
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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.

Jim
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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.
William Hu
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In Rails > 3.2.16, removing the column removes the index.

Mark Swardstrom
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