Ruby-on-rails – Cannot load Ruby EventMachine in Rails console – no such file to load

rename_column :table, :old_column, :new_column

You'll probably want to create a separate migration to do this. (Rename FixColumnName as you will.):

script/generate migration FixColumnName
# creates  db/migrate/xxxxxxxxxx_fix_column_name.rb

Then edit the migration to do your will:

# db/migrate/xxxxxxxxxx_fix_column_name.rb
class FixColumnName < ActiveRecord::Migration
  def self.up
    rename_column :table_name, :old_column, :new_column
  end

  def self.down
    # rename back if you need or do something else or do nothing
  end
end

For Rails 3.1 use:

While, the up and down methods still apply, Rails 3.1 receives a change method that "knows how to migrate your database and reverse it when the migration is rolled back without the need to write a separate down method".

See "Active Record Migrations" for more information.

rails g migration FixColumnName

class FixColumnName < ActiveRecord::Migration
  def change
    rename_column :table_name, :old_column, :new_column
  end
end

If you happen to have a whole bunch of columns to rename, or something that would have required repeating the table name over and over again:

rename_column :table_name, :old_column1, :new_column1
rename_column :table_name, :old_column2, :new_column2
...

You could use change_table to keep things a little neater:

class FixColumnNames < ActiveRecord::Migration
  def change
    change_table :table_name do |t|
      t.rename :old_column1, :new_column1
      t.rename :old_column2, :new_column2
      ...
    end
  end
end

Then just db:migrate as usual or however you go about your business.

For Rails 4:

While creating a Migration for renaming a column, Rails 4 generates a change method instead of up and down as mentioned in the above section. The generated change method is:

$ > rails g migration ChangeColumnName

which will create a migration file similar to:

class ChangeColumnName < ActiveRecord::Migration
  def change
    rename_column :table_name, :old_column, :new_column
  end
end

After much time spinning my wheels I finally have something that is acceptable:

Spork.prefork do
  require "rails/mongoid"
  Spork.trap_class_method(Rails::Mongoid, :load_models)

  # The following does not work correctly with Devise's routes that load the User model. 
  # Results in NameError unintitialized *
  # :reload_routes! triggers :devise_for which loads and caches the User class.
  # https://github.com/timcharper/spork/wiki/Spork.trap_method-Jujutsu
  # require "rails/application"
  # Spork.trap_method(Rails::Application, :reload_routes!) 

  require 'factory_girl_rails'
  Spork.trap_class_method(Factory, :find_definitions)

  require File.expand_path(File.dirname(__FILE__) + '/../config/environment')

end

The problem was mostly due to Rails::Mongoid#load_models. After inspecting the debug output of spork and the backtrace of how things are loaded, some clues are provided as to what is loading the models. This page goes into a bit more detail https://github.com/timcharper/spork/wiki/Spork.trap_method-Jujutsu, but I do not find the :reload_routes! helping with Devise forcing the User class to be preloaded.

My specs are running much faster now. If only I could get my User model to not be cached I would be in bliss, but until then, I shall be mostly satisfied.