Entity framework code-first null foreign key

Question

I have a User < Country model. A user belongs to a country, but may not belong to any (null foreign key).

How do I set this up? When I try to insert a user with a null country, it tells me that it cannot be null.

The model is as follows:

 public class User{
    public int CountryId { get; set; }
    public Country Country { get; set; }
}

public class Country{
    public List<User> Users {get; set;}
    public int CountryId {get; set;}
}

Error: A foreign key value cannot be inserted because a corresponding primary key value does not exist. [ Foreign key constraint name = Country_Users ]"}

Can you kindly correct me if i am wrong. A foreign key is NULLABLE by DEFAULT in code first asp.net mvc - 5 entity framework. — Unbreakable
– Unbreakable, Commented Aug 9, 2017 at 22:34
If we want to make it non nullable. we need to either use fluent api if not then decorate with "Required" attribute. Am I correct? — Unbreakable
– Unbreakable, Commented Aug 9, 2017 at 22:34
If we don't do either then Foreign key will be defaulted to Nullable — Unbreakable
– Unbreakable, Commented Aug 9, 2017 at 22:35

Ladislav Mrnka · Accepted Answer · 2011-04-14 19:49:32Z

218

You must make your foreign key nullable:

public class User
{
    public int Id { get; set; }
    public int? CountryId { get; set; }
    public virtual Country Country { get; set; }
}

answered Apr 14, 2011 at 19:49

Ladislav Mrnka

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12 Comments

Ladislav Mrnka Over a year ago

Virtual is necessary for lazy loading.

Dan VanWinkle Over a year ago

Virtual also adds change tracking, which isn't always wanted. About the only time that we ever want virtual is on collections, but YMMV.

Travis J Over a year ago

@LadislavMrnka - And how would the lazy loading work when an attempt to get a navigation property was made when the id was null (which throws an exception)?

SparK Over a year ago

@TravisJ user.Country returns null then... either catch the exception (optimal) or use if (eww)

BrainSlugs83 Over a year ago

Also -- this doesn't seem to work for Guid based keys. (It makes them nullable, sure, but saving a record to the database with the foreign key set to null fails due to an automatically generated foreign key constraint.) :-(

|

Mark Cooper · Accepted Answer · 2019-03-14 13:04:07Z

15

I prefer this (below):

public class User
{
    public int Id { get; set; }
    public int? CountryId { get; set; }
    [ForeignKey("CountryId")]
    public virtual Country Country { get; set; }
}

Because EF was creating 2 foreign keys in the database table: CountryId, and CountryId1, but the code above fixed that.

edited Mar 14, 2019 at 13:04

Mark Cooper

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answered Sep 17, 2018 at 10:10

user1040323

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1 Comment

perustaja Over a year ago

This is exactly the problem I was having. I don't even have a navigation property in my entity, so it's a bit odd that it's happening anyway.

r3plica · Accepted Answer · 2018-07-24 15:21:11Z

9

I have the same problem now , I have foreign key and i need put it as nullable, to solve this problem you should put

    modelBuilder.Entity<Country>()
        .HasMany(c => c.Users)
        .WithOptional(c => c.Country)
        .HasForeignKey(c => c.CountryId)
        .WillCascadeOnDelete(false);

in DBContext class I am sorry for answer you very late :)

edited Jul 24, 2018 at 15:21

r3plica

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answered Jul 17, 2018 at 17:22

Yaman Melhem

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1 Comment

user1034912 Over a year ago

'WithOptional' no linger valid for EFCore... only has 'WithOne'

Jonathan Moy · Accepted Answer · 2023-09-04 15:54:23Z

5

In EF 7 :

entity.Property(e => e.propertyName).IsRequired(false);

answered Sep 4, 2023 at 15:54

Jonathan Moy

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Comments

Anonimys · Accepted Answer · 2020-01-13 22:26:47Z

0

I recommend to read Microsoft guide for use Relationships, Navigation Properties and Foreign Keys in EF Code First, like this picture.

Guide link below:

https://learn.microsoft.com/en-gb/ef/ef6/fundamentals/relationships?redirectedfrom=MSDN

answered Jan 13, 2020 at 22:26

Anonimys

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