Entity Framework Core 1:n relation causes a loop

I’m using EF Core In-memory database. I have a problem with the definition of the foreign key (navigation property). One user could have multiple tokens. So my user table is called Benutzer and my token table is called RefreshToken:

public class Benutzer
{
    [Key]
    public int BenutzerId { get; set; }

    public string AnmeldeName { get; set; }
    public string Passwort { get; set; }
    public string BevorzugteSprache { get; set; }

    public byte[] Salt { get; set; }

    [NotMapped]
    public int Iterationen { get => 10000; }

    // Navigation property
    // [ForeignKey("BenutzerId")]
    public virtual List<RefreshToken> RefreshTokens { get; set; }
}

public class RefreshToken 
{
    [Key]
    public int RefreshTokenId { get; set; }

    public string Token { get; set; }

    // Navigation property
    public virtual Benutzer Benutzer { get; set; } // <-- caused a loop
    
    [ForeignKey("BenutzerId")]  
    public int BenutzerId { get; set; } 
 }

Without the navigation property in table RefreshToken, it works partially. I can add a token for an user. If I request the user table, I get the attached tokens. But I can add a token with a BenutzerId which is not existing.

Now I added the navigation property to the RefreshToken table, see code above. That has caused a loop.

If I request the user table, I get the attached token, this one attached the user and so on. Further I can add a token with an none existing user.

What’s wrong?

var list = await _dbContext.Set<Benutzer>().ToListAsync();

Thanks a lot

Answers:

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Method 1

The FK definition if on the FK property should point to the navigation property, or if on the navigation property, point to the FK:

public virtual Benutzer Benutzer { get; set; } 
[ForeignKey("Benutzer")]  
public int BenutzerId { get; set; }

or

[ForeignKey("BenutzerId")]  
public virtual Benutzer Benutzer { get; set; } 
public int BenutzerId { get; set; }

For EF Core you don’t actually need the FK Property, you can leave it as a Shadow Property to avoid confusion where you might update a reference (Setting a Benutzer vs. changing BenutzerId)

This should work in EF Core with a shadow property:

[ForeignKey("BenutzerId")]  // No property declared for BenutzerId, EF will treat it behind the scenes.
public virtual Benutzer Benutzer { get; set; }

Method 2

You appear to be missing the action navigation property, you have the FK but not the entity:

public class RefreshToken 
{
    [Key]
    public int RefreshTokenId { get; set; }

    public string Token { get; set; }

    // Navigation property
    public Benutzer Benutzer {get;set;} <<<<<<<<

    [ForeignKey("BenutzerId")]  
    public int BenutzerId { get; set; } 
 }

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