public IEnumerable <Beer> GetByCategories(IEnumerable <Category> categories, int offset, int limit)
        {
            if (categories.Count() == 0)
            {
                return(new List <Beer>());
            }

            return(beerRepository.GetByCategory(categories.Select(c => c.Id), offset, limit).Select(b => ConvertBeerDB(b)));
        }