code: def unique_index_for(keys)
        @unique_index_for_keys_cache ||= {}
        @unique_index_for_keys_cache[keys] if @unique_index_for_keys_cache[keys]
  
        # Take the uniq key having the least number of columns
        @unique_index_for_keys_cache[keys] = uniq_keys_candidates(keys).min_by { |x| x.columns.count }
      end
docs: Finds an index that fits the list of columns (keys) the best
  
   @param keys [Array<Symbol>]
   @raise [Exception] if the unique index for the columns was not found
   @return [ActiveRecord::ConnectionAdapters::IndexDefinition] unique index fitting the keys