compute_cache_version

# File activerecord/lib/active_record/relation.rb, line 482
    def compute_cache_version(timestamp_column) # :nodoc:
      timestamp_column = timestamp_column.to_s

      if loaded?
        size = records.size
        if size > 0
          timestamp = records.map { |record| record.read_attribute(timestamp_column) }.max
        end
      else
        collection = eager_loading? ? apply_join_dependency : self

        with_connection do |c|
          column = c.visitor.compile(table[timestamp_column])
          select_values = "COUNT(*) AS #{model.adapter_class.quote_column_name("size")}, MAX(%s) AS timestamp"

          if collection.has_limit_or_offset?
            query = collection.select("#{column} AS collection_cache_key_timestamp")
            query._select!(table[Arel.star]) if distinct_value && collection.select_values.empty?
            subquery_alias = "subquery_for_cache_key"
            subquery_column = "#{subquery_alias}.collection_cache_key_timestamp"
            arel = query.build_subquery(subquery_alias, select_values % subquery_column)
          else
            query = collection.unscope(:order)
            query.select_values = [select_values % column]
            arel = query.arel
          end

          size, timestamp = c.select_rows(arel, nil).first

          if size
            column_type = model.type_for_attribute(timestamp_column)
            timestamp = column_type.deserialize(timestamp)
          else
            size = 0
          end
        end
      end

      if timestamp
        "#{size}-#{timestamp.utc.to_fs(model.cache_timestamp_format)}"
      else
        "#{size}"
      end
    end