How do you make remove_column reversible?

问题

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.

回答1:

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Anyway, it appears that this documentation is out of date...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.

回答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

来源：https://stackoverflow.com/questions/24520550/how-do-you-make-remove-column-reversible

标签

ruby-on-rails

rails-migrations