code: func (h *Handler) GetProductUsage(password, productID string) (*uint, error) {
  	logger := h.logger.Add("method", "GetProductUsage",
  		"productID", productID)
  
  	return h.uintFromQuery(logger, password,
  		`SELECT SUM(sessions.units_used)
  		   FROM offerings
  		   	JOIN channels
  			ON channels.offering=offerings.id
  			   AND offerings.product=$1
  		   	JOIN sessions
  		     	ON sessions.channel=channels.id`, productID)
  }
docs: // GetProductUsage returns total units used in all channel
  // of all offerings with given product.