FK 和 PK 实体框架核心之间的冲突

Question

I'm trying to build a web app with Entity Framework core, I created two models Category an Pie , I do all things including DbContext an dependency injection, I created a DbInializer class to check if data base is empty and if this is true it will insert some data,the problem is when I run the app I get an exception, as if there is conflict between primary key in Categories table and Foreign key in Pies Table, this is The Exception :

 System.Data.SqlClient.SqlException: The MERGE statement conflicted with the FOREIGN KEY constraint "FK_Pies_Categories_CategoryId".The conflict occurred in database "ShopDb", table "dbo.Categories", column 'CategoryId'.
     at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean & dataReady)
   at System.Data.SqlClient.SqlDataReader.TryHasMoreRows(Boolean & moreRows)
   at System.Data.SqlClient.SqlDataReader.TryHasMoreResults(Boolean & moreResults)
   at System.Data.SqlClient.SqlDataReader.TryNextResult(Boolean & more)
   at System.Data.SqlClient.SqlDataReader.NextResult()
   at Microsoft.EntityFrameworkCore.Update.AffectedCountModificationCommandBatch.Consume(DbDataReader reader)
ClientConnectionId: 775a7294 - 531a - 44cc - 8fbc - 29d293c339d5
         Error Number: 547,State: 0,Class: 16}

This is Pie class :

 public class Pie
 {
        public int PieId { get; set; }
        public string Name { get; set; }
        public string ShortDescrition { get; set; }
        public string LongDescription { get; set; }
        public string AllegryInformation { get; set; }
        public string ImageUrl { get; set; }
        public string ImageThumbnailUrl { get; set; }
        public bool IsPieOfTheWeek { get; set; }
        public bool InStock { get; set; }
        public decimal Price { get; set; }
        public int CategoryId { get; set; }
        public virtual Category Category { get; set; }
}

... here is Category class :

public class Category
{
        public int CategoryId { get; set; }
        public string CategoryName { get; set; }
        public string Description { get; set; }
        public List<Pie> Pies { get; set; }
}

The Configure method of th Startup class:

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
        {
            app.UseDeveloperExceptionPage();
            app.UseStatusCodePages();
            app.UseStaticFiles();
            app.UseMvcWithDefaultRoute();
            DbInitializer.Seed(app);
        }

DbInitializer class :

 public class DbInitializer
 {

        public static void Seed(IApplicationBuilder applicationBuilder)
        {
            AppDbContext context = applicationBuilder.ApplicationServices.GetRequiredService<AppDbContext>();
            if (!context.Categories.Any())
            {
                context.AddRange(
                        new Category { CategoryName = "First Pie", Description="Descriptionslqdfq vdfhsqdqsdhfs qsdhf" },
                        new Category { CategoryName = "Cheese Cackes", Description = "Descriptionslqdfq vdfhsqdqsdhfs qsdhf" },
                        new Category { CategoryName = "Saesonal Pie", Description = "Descriptionslqdfq vdfhsqdqsdhfs qsdhf" });
            }
            if (!context.Pies.Any())
            {
                context.AddRange(new Pie { Name = "Apple Pie", ShortDescrition = "short description", LongDescription = "Long description", AllegryInformation = "sflqjq", ImageUrl = "#", IsPieOfTheWeek = true, InStock = true, Price = 15.65M },
                    new Pie { Name = "Apple Pie", ShortDescrition = "short description", LongDescription = "Long description", AllegryInformation = "sflqjq", ImageUrl = "#", IsPieOfTheWeek = true, InStock = true, Price = 15.65M },
                    new Pie { Name = "Apple Pie", ShortDescrition = "short description", LongDescription = "Long description", AllegryInformation = "sflqjq", ImageUrl = "#", IsPieOfTheWeek = true, InStock = true, Price = 15.65M },
                    new Pie { Name = "Apple Pie", ShortDescrition = "short description", LongDescription = "Long description", AllegryInformation = "sflqjq", ImageUrl = "#", IsPieOfTheWeek = true, InStock = true, Price = 15.65M },
                    new Pie { Name = "Apple Pie", ShortDescrition = "short description", LongDescription = "Long description", AllegryInformation = "sflqjq", ImageUrl = "#", IsPieOfTheWeek = true, InStock = true, Price = 15.65M });
            }
            context.SaveChanges();
        }


 }

Answer 1

This is because you added

public int CategoryId { get; set; }

in pie Entity Framework handles relationship by its self when you add

public virtual Category Category { get; set; }

in your case when you added CategoryId it expects categoryId because its not set to null thats why its giving exception so you can either set CategoryId as nullable int

public int? CategoryId { get; set; }

or you can let entity framework handle it as a nullable for you