Applied fix according to dotnet/efcore#19854 (comment)

Controllers/HomeController.cs

@@ -1,6 +1,12 @@
-using System.Linq;
-using aspnetcoreapp_efcore_inherited_entity_id_problem.Data;
+using aspnetcoreapp_efcore_inherited_entity_id_problem.Data;
 using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.InMemory.Infrastructure.Internal;
+using Microsoft.EntityFrameworkCore.InMemory.Storage.Internal;
+using Microsoft.EntityFrameworkCore.Metadata.Internal;
+using System;
+using System.Linq;
 
 namespace aspnetcoreapp_efcore_inherited_entity_id_problem.Controllers
 {
@@ -18,28 +24,48 @@ public HomeController(AppDbContext dbContext)
 
         public IActionResult Index()
         {
+            // Fix for EF Core 3 derived entity types
+            int maxId = _dbContext.Animal.Max(x => x.Id);
+            if (maxId == 2)
+            {
+                BumpGenerator(_dbContext, typeof(AnimalBase), nameof(AnimalBase.Id), 2);
+            }
+
             // Uncomment the case you would like to test...
 
             // 1st problem:
             // This causes error "ArgumentException: An item with the same key has already been added. Key: 1".
             _dbContext.Cat.Add(
-                    new Cat
-                    {
-                        Name = "Tom",
-                    });
+                   new Cat
+                   {
+                       Name = "Tom",
+                   });
             _dbContext.SaveChanges();
 
             // 2nd problem:
             // This inserts new Dog with id 1 and replaces Cat with id 1.
-            //_dbContext.Dog.Add(
-            //    new Dog
-            //    {
-            //        Name = "Laika"
-            //    });
-            //_dbContext.SaveChanges();
-
+            _dbContext.Dog.Add(
+                new Dog
+                {
+                    Name = "Laika"
+                });
+            _dbContext.SaveChanges();
 
             return View(_dbContext.Animal.ToList());
         }
+
+        private static void BumpGenerator(
+            DbContext context, Type entityClrType, string propertyName, int newLowBound)
+        {
+            var entityType = context.Model.FindEntityType(entityClrType);
+            var property = entityType.FindProperty(propertyName);
+            var options = context.GetService<IDbContextOptions>().FindExtension<InMemoryOptionsExtension>();
+            var inMemoryStore = context.GetService<IInMemoryStoreCache>().GetStore(options.StoreName);
+            var generator = inMemoryStore.GetIntegerValueGenerator<int>(property);
+
+            var values = new object[entityType.GetDeclaredProperties().Count()];
+            values[property.GetIndex()] = newLowBound;
+            generator.Bump(values);
+        }
     }
 }

README.md

@@ -1 +1,3 @@
-"# aspnetcoreapp-efcore-inherited-entity-id-problem" 
+ASP NET Core demo app to demonstarte problem in EF Core 3:
+InMemory - bad key management for derived entity types
+https://github.com/dotnet/efcore/issues/19854