Rails: How to add add_index to existing table

Question

I already migrated a table called units with several columns. I was wondering how to migrate in a stand alone 'add_index' to this table using the cmd. Is this code correct:

class AddIndexToUnits < ActiveRecord::Migration
  def self.up
    add_index :units, :lesson_id
  end

  def self.down
    remove :units
  end
end

I have a feeling the self.down could be wrong, I am unsure.

Kris · Accepted Answer · 2011-05-24 09:29:44Z

14

The self.up method is correct. Use this for your self.down:

remove_index :units, :column => :lesson_id

answered May 24, 2011 at 9:29

Kris

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2 Comments

yoshyosh Over a year ago

thanks :D, seems like you guys have slightly different answers

Kris Over a year ago

The difference is that Yule gives a name to the index in his self.up which he uses for removing it. The code I provided removes the index based on its column name.

Yule · Accepted Answer · 2011-05-24 09:31:12Z

10

Almost

class AddIndexToUnits < ActiveRecord::Migration
  def self.up
    add_index :units, :lesson_id, :name=>'lesson_index'
  end

  def self.down
    remove_index :units, 'lesson_index'
  end
end

answered May 24, 2011 at 9:31

Yule

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4 Comments

yoshyosh Over a year ago

thanks! :) how do I add this from the command line: rails generate migration add_index_to_units doesn't work

Arsen7 Over a year ago

What exactly does not work? A new migration file is not created?

yoshyosh Over a year ago

a file is created but it looks really weird... like has 2 add_columns in the self.up definition

Yule Over a year ago

@Arsen I'm not sure you can do this directly from command line. Is there a reason you can't just edit migration file? If not checkout github.com/capotej/migration_for. Seems to do what you're after...

Laas · Accepted Answer · 2011-05-24 09:39:08Z

2

To remove an index, you must use remove_index with the same table and column specification as the self.up's add_index has. So:

def self.down
  remove_index :units, :lesson_id
end

A multi-column index example would be:

def self.down
  remove_index :units, [:lesson_id, :user_id]
end

answered May 24, 2011 at 9:39

Laas

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