Temporary index name too long in Rails migration

Question

I've got a problem trying to rollback one of my migration. It seems as if Rails is generating a temporary table for the migration, with temporary indices. My actual index on this table is less than 64 characters, but whenever Rails tries to create a temporary index for it, it turns into a name longer than 64 characters, and throws an error.

Here's my simple migration:

class AddColumnNameToPrices < ActiveRecord::Migration
  def self.up
     add_column :prices, :column_name, :decimal
  end

  def self.down
    remove_column :prices, :column_name
  end
end

Here's the error I'm getting:

==  AddColumnNameToPrices: reverting ============================================
-- remove_column(:prices, :column_name)
rake aborted!
An error has occurred, this and all later migrations canceled:

Index name 'temp_index_altered_prices_on_column_and_other_column_and_third_column' on table     'altered_prices' is too long; the limit is 64 characters

I've changed the column names, but the example is still there. I can just make my change in a second migration, but that still means I can't rollback migrations on this table. I can rename the index in a new migration, but that still locks me out of this single migration.

Does anyone have ideas on how to get around this problem?

Answer 1

It looks like your database schema actually has index called prices_on_column_and_other_column_and_third_column. You have probably defined the index in your previous play with migrations. But than just removed index definition from migrations.

If it is true you have 2 options:

• The simplier one (works if you code is not in production). You can recreate database from scratch using migrations (not from db/schema.rb) by calling rake db:drop db:create db:migrate. Make sure that you do not create this index with long name in other migration files. If you do, add :name => 'short_index_name' options to add_index call to make rails generate shorter name for the index.
• If you experience this problem on a production database it is a bit more complicated. You might need to manually drop the index from the database console.