Добавил миграцию для создания базы данных

MyCities/Migrations/20190619144333_InitialMigration.Designer.cs

+// <auto-generated />
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+using MyCities.Models;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+
+namespace MyCities.Migrations
+{
+    [DbContext(typeof(MyCitiesDbContext))]
+    [Migration("20190619144333_InitialMigration")]
+    partial class InitialMigration
+    {
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder
+                .HasAnnotation("Npgsql:ValueGenerationStrategy", NpgsqlValueGenerationStrategy.SerialColumn)
+                .HasAnnotation("ProductVersion", "2.1.1-rtm-30846")
+                .HasAnnotation("Relational:MaxIdentifierLength", 63);
+
+            modelBuilder.Entity("MyCities.Models.City", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<string>("Country");
+
+                    b.Property<string>("Name");
+
+                    b.Property<int>("Population");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("Cities");
+                });
+
+            modelBuilder.Entity("MyCities.Models.Person", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<string>("AvatarImagePath");
+
+                    b.Property<string>("FullName");
+
+                    b.Property<string>("NickName");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("Persons");
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}

MyCities/Migrations/20190619144333_InitialMigration.cs

+using Microsoft.EntityFrameworkCore.Migrations;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+
+namespace MyCities.Migrations
+{
+    public partial class InitialMigration : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "Cities",
+                columns: table => new
+                {
+                    Id = table.Column<int>(nullable: false)
+                        .Annotation("Npgsql:ValueGenerationStrategy", NpgsqlValueGenerationStrategy.SerialColumn),
+                    Name = table.Column<string>(nullable: true),
+                    Country = table.Column<string>(nullable: true),
+                    Population = table.Column<int>(nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_Cities", x => x.Id);
+                });
+
+            migrationBuilder.CreateTable(
+                name: "Persons",
+                columns: table => new
+                {
+                    Id = table.Column<int>(nullable: false)
+                        .Annotation("Npgsql:ValueGenerationStrategy", NpgsqlValueGenerationStrategy.SerialColumn),
+                    FullName = table.Column<string>(nullable: true),
+                    NickName = table.Column<string>(nullable: true),
+                    AvatarImagePath = table.Column<string>(nullable: true)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_Persons", x => x.Id);
+                });
+        }
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "Cities");
+
+            migrationBuilder.DropTable(
+                name: "Persons");
+        }
+    }
+}

MyCities/Migrations/MyCitiesDbContextModelSnapshot.cs

+// <auto-generated />
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+using MyCities.Models;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+
+namespace MyCities.Migrations
+{
+    [DbContext(typeof(MyCitiesDbContext))]
+    partial class MyCitiesDbContextModelSnapshot : ModelSnapshot
+    {
+        protected override void BuildModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder
+                .HasAnnotation("Npgsql:ValueGenerationStrategy", NpgsqlValueGenerationStrategy.SerialColumn)
+                .HasAnnotation("ProductVersion", "2.1.1-rtm-30846")
+                .HasAnnotation("Relational:MaxIdentifierLength", 63);
+
+            modelBuilder.Entity("MyCities.Models.City", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<string>("Country");
+
+                    b.Property<string>("Name");
+
+                    b.Property<int>("Population");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("Cities");
+                });
+
+            modelBuilder.Entity("MyCities.Models.Person", b =>
+                {
+                    b.Property<int>("Id")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<string>("AvatarImagePath");
+
+                    b.Property<string>("FullName");
+
+                    b.Property<string>("NickName");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("Persons");
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}

MyCities/Models/City.cs

@@ -2,6 +2,7 @@ namespace MyCities.Models
 {
     public class City
     {
+        public int Id { get; set; }
         public string Name { get; set; }
         public string Country { get; set; }
         public int Population { get; set; }

MyCities/Models/MyCitiesDbContext.cs

+using Microsoft.EntityFrameworkCore;
+
+namespace MyCities.Models
+{
+    public class MyCitiesDbContext : DbContext
+    {
+        public DbSet<City> Cities { get; set; }
+        public DbSet<Person> Persons { get; set; }
+
+        public MyCitiesDbContext(DbContextOptions<MyCitiesDbContext> options) : base(options)
+        {
+
+        }
+    }
+}

MyCities/Models/Person.cs

@@ -2,6 +2,7 @@ namespace MyCities.Models
 {
     public class Person
     {
+        public int Id { get; set; }
         public string FullName { get; set; }
         public string NickName { get; set; }
         public string AvatarImagePath { get; set; }

MyCities/MyCities.csproj

@@ -6,6 +6,9 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.AspNetCore.App" />
+    <PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL">
+      <Version>2.1.1.1</Version>
+    </PackageReference>
   </ItemGroup>
 
 </Project>

MyCities/Startup.cs

-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Http;
-using Microsoft.AspNetCore.HttpsPolicy;
 using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using MyCities.Models;
@@ -33,6 +29,9 @@ namespace MyCities
                 options.MinimumSameSitePolicy = SameSiteMode.None;
             });
 
+            string connectionString = Configuration.GetConnectionString("Default");
+            services.AddDbContext<MyCitiesDbContext>(options => options.UseNpgsql(connectionString));
+
             services.AddSingleton<ICityRepository, InMemoryCityRepository>();
             services.AddSingleton<IPersonRepository, InMemoryPersonRepository>();
             services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

MyCities/appsettings.json

 {
+  "ConnectionStrings": {
+    "Default": "Host=localhost;Database=MyCities;Username=postgres;Password=qwerty"
+  },
   "Logging": {
     "LogLevel": {
       "Default": "Warning"