(from my blog post)

As I posted here, Greg helped me find a way to override the way Rails migrations create Oracle sequences by putting code in the environment.rb file. I mentioned at the bottom of that post that I would like to find a way to make it also support Oracle’s bitmap indexes. This turned out to be trivial, simply a matter of adding another section to the override code I already had. The method which creates the indexes is ActiveRecord::ConnectionAdapters::SchemaStatements.add_index. It surprises me that there isn’t an Oracle-specific override in the OracleAdapter. Anyway, my code from the last blog post already had the ActiveRecord::ConnectionAdapters in scope, so I added another section as below:

module SchemaStatements
  def add_index(table_name, column_name, options = })
    column_names = Array(column_name)
    index_name   = index_name(table_name, :column => column_names)

    if Hash === options # legacy support, since this param was a string
      index_type = options[:unique] ? “UNIQUE” : options[:bitmap] ? “BITMAP” : “”
      index_name = options[:name] || index_name
    else
      index_type = options
    end
    quoted_column_names = column_names.map { |e| quote_column_name(e) }.join(”, “)
    execute “CREATE #{index_type} INDEX #{quote_column_name(index_name)} ON #{quote_table_name(table_name)} (#{quoted_column_names})”
  end
end

Only one change was made to the original:

index_type = options[:unique] ? “UNIQUE” : “”

became

index_type = options[:unique] ? “UNIQUE” : options[:bitmap] ? “BITMAP” : “”

This now allows the specification of a bitmap option to add_index in the migration file:

add_index "mytable", ["mycolumn"], :name => “mytable_bitmap_idx”, :bitmap => true

I’m pretty happy with the ability Ruby on Rails gives you to customize things like this. Being able to override class methods in this way is really pretty special.