Monkey patch for ActiveRecord::CollectionCacheKey

collection_cache_key.rb

# MONKEY_PATCH: Override ActiveRecord::CollectionCacheKey#collection_cache_key
#               This allows us to preserve existing selects when paginating
#               thus preventing errors when ordering by custom/computed select fields
#
# For the original implementation SEE: https://github.com/rails/rails/blob/5-0-4/activerecord/lib/active_record/collection_cache_key.rb
# IMPORTANT: Upgrade this file whenever upgrading rails versions
module ActiveRecord::CollectionCacheKey
  def collection_cache_key(collection = all, timestamp_column = :updated_at) # :nodoc:
    query_signature = Digest::MD5.hexdigest(collection.to_sql)
    key = "#{collection.model_name.cache_key}/query-#{query_signature}"

    if collection.loaded?
      size = collection.size
      if size > 0
        timestamp = collection.max_by(&timestamp_column)._read_attribute(timestamp_column)
      end
    else
      column_type = type_for_attribute(timestamp_column.to_s)
      column = "#{connection.quote_table_name(collection.table_name)}.#{connection.quote_column_name(timestamp_column)}"
      select_values = "COUNT(*) AS #{connection.quote_column_name("size")}, MAX(%s) AS timestamp"

      if collection.limit_value || collection.offset_value
        query = collection.spawn
        subquery_alias = "subquery_for_cache_key"
        subquery_column_alias = "subquery_for_cache_key_timestamp_column"
        query.select_values = [query.select_values.dup, "#{column} AS #{subquery_column_alias}"].flatten
        subquery = query.arel.as(subquery_alias)
        arel = Arel::SelectManager.new(subquery).project(select_values % subquery_column_alias)
      else
        query = collection.unscope(:order)
        query.select_values = [select_values % column]
        arel = query.arel
      end

      result = connection.select_one(arel, nil, query.bound_attributes)

      if result.blank?
        size = 0
        timestamp = nil
      else
        size = result["size"]
        timestamp = column_type.deserialize(result["timestamp"])
      end

    end

    if timestamp
      "#{key}-#{size}-#{timestamp.utc.to_s(cache_timestamp_format)}"
    else
      "#{key}-#{size}"
    end
  end
end