Nullable / Optional Foreign Key using EF Core 5

Question

Hi All,

I have been struggling to find an answer to having a nullable / optional foreign key using .net core 5 and EF core 5.

The relationship is a one-to-many and the parent table does not always require the foreign key making it optional.

I am migrating a desktop app to a .net core 5 web app so my DB parent table has a majority of null values for the foreign key column.

The problem comes in when I want to view any parent table records with a null foreign key value as EF Core simply ignores the records and does not display anything.

When the parent table foreign key value is populated the records show up.

I have tried to simply make the foreign key nullable in the model as below but I get a null exception error at run time.

[ForeignKey("CesArea")]

public int? AreaID { get; set;

public virtual CesArea CesArea { get; set; }

Any help would be greatly appreciated!

Rijwan Ansari · Answer

Add below in your OnModelCreating method of context class. modelBuilder.Entity () .WithOptional(c => c.CesArea) .HasForeignKey(c => c.AreaID) .WillCascadeOnDelete( false );

Sachin Singh · Answer

At first your code should work for 100% , so, show us both the entity , so that we could get what went wrong. use fluentAPI protected override void OnModelCreating(DbModelBuilder mb) { mb.Entity .HasMany(o1 => o1.ceaArea) .WithOptional(o2 => o2.MainEntity); base .OnModelCreating(mb); } https://www.sharpencode.com/article/EntityFramework/how-to-establish-relationship-correctly/using-fluent-api https://www.sharpencode.com/article/EntityFramework/how-to-establish-relationship-correctly/using-data-annotation https://www.sharpencode.com/article/EntityFramework/overriding-code-first-convention

Brian Turner · Answer

I tried your suggestions but DbModelBuilder is not available. What I do have is ModelBuilder but niether options HasMany or WithOption is available. Below is an example with the assemblies I am using. using System; using System.Collections.Generic; using System.Linq; using System.Threading.Tasks; using Microsoft.EntityFrameworkCore; using System.ComponentModel; using System.ComponentModel.DataAnnotations; using System.ComponentModel.DataAnnotations.Schema; using Microsoft.AspNetCore.Mvc; using CorpCareApp.Models; namespace CorpCareApp.Data { public class CorpCareAppContext : DbContext { public CorpCareAppContext (DbContextOptions options) : base(options) { } protected override void OnModelCreating(ModelBuilder modelBuilder) { modelBuilder.Entity }