How to create a migration to remove an index only if it exists, rather than throwing an exception if it doesn't?

Question

Right now, the current migration might fail, if the books table doesn\'t have created_at or updated_at fields:

class AddTi         


        

Answer 1

You can use the index_exists? method within your migration to test whether the index you need to remove is actually there.

Take a look at the documentation here: http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/index_exists%3F

I've not tested it, but you should be able to use something like this:

class AddTimestampIndexes < ActiveRecord::Migration
  def up
    remove_index :books, :created_at if index_exists?(:books, :created_at)
    remove_index :books, :updated_at if index_exists?(:books, :updated_at)

    add_index  :books, :created_at
    add_index  :books, :updated_at
  end

  def down
    remove_index :books, :created_at
    remove_index :books, :updated_at
  end
end

Although, by the looks of things, you really only want to create them if they don't exist? This might be more appropriate for your migration:

class AddTimestampIndexes < ActiveRecord::Migration
  def up
    add_index  :books, :created_at unless index_exists?(:books, :created_at)
    add_index  :books, :updated_at unless index_exists?(:books, :updated_at)
  end

  def down
    remove_index :books, :created_at
    remove_index :books, :updated_at
  end
end