How do you make remove_column reversible?

Question

I have a migration that removes a column:

def change
  remove_column :foos, :bar, :boolean
end

When I try to rake db:rollback that migration, I get the following error:

remove_column is only reversible if given a type.

The ActiveRecord::Migration documentation says that the following is the signature for remove_column:

remove_column(table_name, column_name, type, options)

So my type in this case should be :boolean, and I expect that migration to be reversible. What am I missing?

I can certainly break this out into an up and down migration to avoid this problem, but I'd like to understand why the change syntax isn't working in this case.

I'm using Rails 4.1.2 in this case, and that matches the version listed in the documentation. — , Jul 01 '14 at 23:12
def up; remove_column :foos, :bar, :boolean; end; for the migrate and rollback will call down. def down; add_column :foos, :bar, :boolean; end; — Afonso Tsukamoto, Jul 01 '14 at 23:12
I have this irreversible migration problem as well, and I have already ran rake:db migrate. I can't rollback and edit, so I'm thinking I'll just delete with rails destroy migration migration_name and redo the migration with the column_type. I'll let you know how it turns out — tomb, Feb 06 '18 at 19:44
@tomb ah I am more than 2 years late and I think the steps you mentioned must have messed up your database even more ): I hope you managed to resolve it all. That's not how you do it. — ARK, Jul 14 '20 at 15:34
@ARK better late than never? I did manage to resolve it, and posted my steps here: https://stackoverflow.com/a/51061650/1072655 — tomb, Jul 22 '20 at 23:20

score 103 · Accepted Answer · edited Jun 20 '20 at 09:12

Simply adding the 3rd argument (the column's :type) to the remove_column method makes that migration reversible. So the OP's original code actually did work, as in:

remove_column :foos, :bar, :boolean

The rest of this answer was an attempt to discover why this method would not have been working, but the OP ended up getting it to work.

I see somewhat contrary info in the documentation for ActiveRecord::Migration:

Some commands like remove_column cannot be reversed. If you care to define how to move up and down in these cases, you should define the up and down methods as before.

For a list of commands that are reversible, please see ActiveRecord::Migration::CommandRecorder.

And this from ActiveRecord::Migration::CommandRecorder:

ActiveRecord::Migration::CommandRecorder records commands done during a migration and knows how to reverse those commands. The CommandRecorder knows how to invert the following commands:

add_column

add_index

add_timestamps

create_table

create_join_table

remove_timestamps

rename_column

rename_index

rename_table

Anyway, it appears that this documentation is out of date... Digging into the source on github:

The method that's giving you grief is:

def invert_remove_column(args)
  raise ActiveRecord::IrreversibleMigration, "remove_column is only reversible if given a type." if args.size <= 2
  super
end

I gave this a shot... setup a migration on my Rails 4.1.2 app and the migration worked both ways -- up and down. Here was my migration:

class TestRemoveColumn < ActiveRecord::Migration
  def change
    remove_column :contacts, :test, :boolean
  end
end

I also tried with the :boolean argument missing and got the same error as you're talking about. Are you sure you're on the final version of Rails 4.1.2 -- not one of the release candidates? If you are, I'd suggest putting a binding.pry into the Rails source for the invert_remove_column method to inspect the arguments list and see what's going on. To do so, just run bundle open activerecord and then explore to: lib/active_record/migration/command_recorder.rb:128.

I had some wires crossed somewhere in my Rails code, and am no longer able to reproduce my problem. I think you might have been right that I may have been using a strange Rails version (despite having 4.1.2 in my gemfile). In any case, I think this is a great answer for helping future readers troubleshoot their own oddities if anyone else runs into similar problems. — , Jul 03 '14 at 17:42
Great research to try to discover what might be causing the error. Congrats! — Angelos Makrygiorgos, Mar 17 '17 at 10:19
This indeed works, but there's a HUGE caveat here: if you are removing a column that contains a reference (like `user_id`), you probably have an index on that column, which will be automatically removed by Rails 4+ on `remove_column`. If you revert this migration, even if you specify the column type (like `:integer`), it will NOT re-add the index on the column, which can bring you a huge performance problem. So I would recommend spending a little bit more time and writing the up and down methods on the migration. — sandre89, Sep 10 '18 at 13:43

score 2 · Answer 2 · answered Jul 02 '14 at 00:24

2

Instead of using change, you use up and down methods to your migration:

def up
  remove_column :foos, :bar
end

def down
  add_column :foos, :bar, :boolean
end

answered Jul 02 '14 at 00:24

bratsche

2,666
20
23

score 0 · Answer 3 · answered Jan 06 '23 at 07:13

0

If you're doing a bulk remove of columns, you can make the migration reversible as follows (since rails 6.1)


def change
    change_table :foobar, bulk: true do |t|
      t.remove :foo, type: :float
      t.remove :bar, type: :int
    end
  end

answered Jan 06 '23 at 07:13

Bruno Degomme

883
10
11

How do you make remove_column reversible?

3 Answers3

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