code: def uniq_keys_candidates(keys)
        # Find all uniq indexes that that are covering our keys
        uniq_key_candidates = unique_indexes.each_with_object([]) { |i, obj| obj << i if (keys - i.columns.map(&:to_sym)).empty? }
  
        if unique_indexes.blank? || uniq_key_candidates.blank?
          raise "#{self} and its table #{model_class.table_name} must have a unique index defined "\
                  "covering columns #{keys} to be able to use saver_strategy :concurrent_safe_batch."
        end
  
        uniq_key_candidates
      end
docs: Find candidates for unique key. Candidate must cover all columns we are passing as keys.
  
   @param keys [Array<Symbol>]
   @raise [Exception] if the unique index for the columns was not found
   @return [Array<ActiveRecord::ConnectionAdapters::IndexDefinition>] Array of unique indexes fitting the keys