public async Task CategoryIdByNameTest()
        {
            var options = new DbContextOptionsBuilder <ApplicationDbContext>()
                          .UseInMemoryDatabase(databaseName: Guid.NewGuid().ToString()).Options;
            var dbContext = new ApplicationDbContext(options);

            dbContext.Categories.Add(new Category
            {
                Id   = 1,
                Name = "First",
            });
            dbContext.Categories.Add(new Category
            {
                Id   = 2,
                Name = "Second",
            });
            await dbContext.SaveChangesAsync();

            var repository = new EfDeletableEntityRepository <Category>(dbContext);
            var service    = new CategoriesService(repository);

            Assert.Equal(1, service.CategoryIdByName("First"));
            Assert.Equal(2, service.CategoryIdByName("Second"));
        }