Documentation should include how to map many to many relationship with self-referencing relations

[prashantchoudhary]

Many to many relations don't work by default, if it is a reference between same entity. And implementing this is not trivial. So I believe the work around should be included in documentation.

//Person class
public partial class Person
{
    public Guid Id {get; private set;}
    private readonly List<Friendship> _allFriends = new();
    public IReadOnlyCollection<Friendship> AllFriends => _allFriends;
}

//Friendship class
public class Friendship
{
    public Guid Id { get; set; }
    public Person Friend { get; init; }
    public Friendship(Person friend)
    {
        Friend = friend;
    }
}
//DbContext
modelBuilder.Entity<Friendship>().HasOne(friendship => friendship.Friend).WithMany();
modelBuilder.Entity<Person>().HasMany(p => p.Friends).WithOne(f => f.Friend);

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Document Details

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• ID: 24b2b50e-35f8-9e0b-8ce5-840e1ea816e6
• Version Independent ID: 9e0e6e2c-48fa-76c3-3bd9-813a6df22645
• Content: Relationships - EF Core
• Content Source: entity-framework/core/modeling/relationships.md
• Product: entity-framework
• Technology: entity-framework-core
• GitHub Login: @AndriySvyryd
• Microsoft Alias: avickers

[ajcvickers]

@prashantchoudhary

Many to many relations don't work by default, if it is a reference between same entity

Please provide more details on this. Ideally, attach a small, runnable project or post a small, runnable code listing that reproduces what you are seeing so that we can investigate.

[prashantchoudhary]

Adding the link to Gist.
Link to the original issue with workaround: dotnet/efcore#27785

[ajcvickers]

@prashantchoudhary So are you attempting to use EF mappings to create a many-to-many relationship between Person and Person, using Friendship as the join table? Or are you attempting to create two one-to-many relationships to simulate a many-to-many--i.e. not using EF's many-to-many mapping support? Also, are you attempting to use the same FK and navigation for both directions of the many-to-many? (I'm trying to understand your code, but I'm finding it really difficult to understand the intention here.)

[prashantchoudhary]

I am trying to represent friendship, Where One person can have many Friendships. So it is Many to many relation, but not direct.

Person--> HasMany -FriendShip
Friendship --> HasOne-Person

So yes, it is many to many relations between Persons using Friendship as the Join table.
Navigation is Unidirectional. So if two Persons are friends then there will be two entries in the Frienship Table..
Sorry if my code was not readable. Will see if it can be refactored.

[slch]

In docs there are no examples of
Self-referencing relationship: A relationship in which the dependent and the principal entity types are the same.
while it being mentioned.
Addition of such examples with self-referencing X-to-X relationships would close this issue.

[ajcvickers]

@prashantchoudhary I think the main issue here is that EF cannot re-use the same navigation property for multiple relationships. That being said, we will document the normal way to do self-referencing many-to-many relationships.