From http://penguincoder.org/pages/A_Slightly_Better_Way_To_Find_Missing_Foreign_Key_Indexes

example output

    add_index :attachments, :parent_id
    add_index :attachments, :asset_id
    add_index :domain_names, :user_id
    add_index :domain_names, :event_id
    add_index :event_memberships, :user_id
    add_index :event_memberships, :event_id
    remove_index :attachments, :parent_id
    remove_index :attachments, :asset_id
    remove_index:domain_names, :user_id
    remove_index:domain_names, :event_id
    remove_index:event_memberships, :user_id
    remove_index:event_memberships, :event_id

potentially_missing_indexes.rb

missing = {}
c = ActiveRecord::Base.connection
c.tables.collect do |t|
  columns = c.columns(t).collect(&:name).select { |x| x.ends_with?  ("_id" || x.ends_with("_type")) }
  indexed_columns = c.indexes(t).collect(&:columns).flatten.uniq
  unindexed = columns - indexed_columns
  missing[t] = unindexed unless unindexed.empty?
end

missing.each do |table, columns|
  columns.each do |col|
    puts "    add_index :#{table}, :#{col}"
  end
end
missing.each do |table, columns|
  columns.each do |col|
    puts "    remove_index :#{table}, :#{col}"
  end
end

The original article linked at the top does not seem to be online anymore.
This gist ist for finding missing indixes for your rails db tables. It also lists the contents of a possible migration for adding them.

Found the article back in the days with archive.org. It only contains the script. https://web.archive.org/web/20090928080237/http://penguincoder.org/pages/A_Slightly_Better_Way_To_Find_Missing_Foreign_Key_Indexes

A further article is linked there:
https://tomafro.net/2009/09/quickly-list-missing-foreign-key-indexes