6

User-Friend relationship

I find an answer

Entity Framework Core: many-to-many relationship with same entity and try like this.

Entitys:

public class User
{
    public int UserId { get; set; }

    public virtual ICollection<Friend> Friends { get; set; }
}

public class Friend
{
    public int MainUserId { get; set; }

    public User ManUser { get; set; }

    public int FriendUserId { get; set; }

    public User FriendUser { get; set; }
}

The fluent API:

modelBuilder.Entity<Friend>()
    .HasKey(f => new { f.MainUserId, f.FriendUserId });

modelBuilder.Entity<Friend>()
    .HasOne(f => f.ManUser)
    .WithMany(mu => mu.Friends)
    .HasForeignKey(f => f.MainUserId);

modelBuilder.Entity<Friend>()
    .HasOne(f => f.FriendUser)
    .WithMany(mu => mu.Friends)
    .HasForeignKey(f => f.FriendUserId);

When I Add-Migration, the error message is

Cannot create a relationship between 'User.Friends' and 'Friend.FriendUser', because there already is a relationship between 'User.Friends' and 'Friend.ManUser'. Navigation properties can only participate in a single relationship.

What should I do? Or I should create an Entity FriendEntity:User?

Community
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Lin Baby
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2 Answers2

9

The problem is that you can't have one collection to support both one-to-many associations. Friend has two foreign keys that both need an inverse end in the entity they refer to. So add another collection as inverse end of MainUser:

public class User
{
    public int UserId { get; set; }
    public virtual ICollection<Friend> MainUserFriends { get; set; }
    public virtual ICollection<Friend> Friends { get; set; }
}

And the mapping:

modelBuilder.Entity<Friend>()
    .HasKey(f => new { f.MainUserId, f.FriendUserId });

modelBuilder.Entity<Friend>()
    .HasOne(f => f.MainUser)
    .WithMany(mu => mu.MainUserFriends)
    .HasForeignKey(f => f.MainUserId).OnDelete(DeleteBehavior.Restrict);

modelBuilder.Entity<Friend>()
    .HasOne(f => f.FriendUser)
    .WithMany(mu => mu.Friends)
    .HasForeignKey(f => f.FriendUserId);

One (or both) of the relationships should be without cascading delete to prevent multiple cascade paths.

Gert Arnold
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5

It's not mandatory the second collection. You only need to left de .WithMany() empty like this:

modelBuilder.Entity<Friend>()
    .HasOne(f => f.MainUser)
    .WithMany()
    .HasForeignKey(f => f.MainUserId);

modelBuilder.Entity<Friend>()
    .HasOne(f => f.FriendUser)
    .WithMany()
    .HasForeignKey(f => f.FriendUserId);

look at this : https://github.com/aspnet/EntityFramework/issues/6052

Alejo Ferrand
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  • Thanks, leaving the WithMany seems to have resolved the issue for me. – Gary Holland Aug 25 '17 at 09:32
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    Trying this left me with an extra unnecessary foreign key relationship in the DB (I had three, instead of two. One of which was redundant, but just named differently). To fix it, I re-added one of parameter to the WithMany() call as follow: modelBuilder.Entity() .HasOne(f => f.FriendUser) .WithMany(mu => mu.MainUserFriends) .HasForeignKey(f => f.FriendUserId); – BernardV Nov 10 '17 at 02:41