Entity framework code-first null foreign key

entity-framework mapping ef-code-first foreign-key-relationship entity-framework-4.1

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.

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?

If we don't do either then Foreign key will be defaulted to Nullable

Ladislav Mrnka

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; }
}

Virtual is necessary for lazy loading.

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

@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)?

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

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

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.

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

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 :)

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

Anonimys

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

https://i.stack.imgur.com/G2bKa.png

Guide link below:

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

Entity framework code-first null foreign key

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