Добавил реализацию репозитория для работы с БД

MyCities/Repositories/CityRepository.cs

+using System.Collections.Generic;
+using System.Linq;
+using MyCities.Models;
+
+namespace MyCities.Repositories
+{
+    public class CityRepository : ICityRepository
+    {
+        private MyCitiesDbContext _dbContext;
+
+        public List<City> Cities => _dbContext.Cities.ToList();
+
+        public CityRepository(MyCitiesDbContext dbContext)
+        {
+            _dbContext = dbContext;
+        }
+
+        public void Add(City city)
+        {
+            _dbContext.Cities.Add(city);
+            _dbContext.SaveChanges();
+        }
+    }
+}

MyCities/Startup.cs

@@ -32,8 +32,8 @@ namespace MyCities
             string connectionString = Configuration.GetConnectionString("Default");
             services.AddDbContext<MyCitiesDbContext>(options => options.UseNpgsql(connectionString));
 
-            services.AddSingleton<ICityRepository, InMemoryCityRepository>();
-            services.AddSingleton<IPersonRepository, InMemoryPersonRepository>();
+            services.AddTransient<ICityRepository, CityRepository>();
+            services.AddTransient<IPersonRepository, InMemoryPersonRepository>();
             services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
         }