Nullable / Optional Foreign Key using EF Core 5

Brian Turner

5y19.6k.NET

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!

3 Replies

Know the answer? Post it — somebody with the same question will find it here.

Brian Turner5y ago
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

}
Rijwan Ansari5y ago
Add below in your OnModelCreating method of context class.
modelBuilder.Entity()
     .WithOptional(c => c.CesArea)
     .HasForeignKey(c => c.AreaID)
     .WillCascadeOnDelete(false);
Sachin Singh5y ago
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