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 AddTimestampIndexes < ActiveRecord::Migration   def up     remove_index :books, :created_at     remove_index :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 

Does remove_index take any options to silently proceed if it fails to remove the index rather than raising an error?

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 

Answer 2

There is also index_name_exists?(table_name, index_name) method which let's you check for an index by it's name. It's helpful for checking for existence of multi-column indexes.

Documentation - index_name_exists