def to_hash
      result = { }

      # id
      result['_id'] = @_id if @_id

      # timestamp
      result['at'] = @at

      # kind
      result['kind'] = kind.to_s

      # entities
      @entities.each do |entity_name, entity|
        result[entity_name.to_s] = entity.model_id
      end

      # meta
      result['meta'] = @meta.stringify_keys unless @meta.blank?

      result
    end
def to_hash
      result = { }

      # id
      result['_id'] = @_id if @_id

      # timestamp
      result['at'] = @at

      # kind
      result['kind'] = kind.to_s

      # entities
      @entities.each do |entity_name, entity|
        result[entity_name.to_s] = entity.model_id
      end

      # meta
      result['meta'] = @meta.stringify_keys unless @meta.blank?

      result
    end
Serialize activity to a hash

 @note All keys are stringified (ie. there is no Symbol)

 @return [Hash] Activity hash