Cannot create siblings relation between same models

[LinusGeffarth]

I want to create a siblings relationship between two User models, for a sort-of "friend" feature, similar to Facebook.
I created the UserFriendsPivot:

final class UserFriendsPivot: MySQLPivot, ModifiablePivot {
    var id: Int?
    var userID: User.ID
    var friendID: User.ID
    
    typealias Left = User
    typealias Right = User
    
    static var leftIDKey: WritableKeyPath<UserFriendsPivot, Int> {
        return \.userID
    }
    
    static var rightIDKey: WritableKeyPath<UserFriendsPivot, Int> {
        return \.friendID
    }
    
    init(_ user: User, _ friend: User) throws {
        self.userID   = try user  .requireID()
        self.friendID = try friend.requireID()
    }
}

extension UserFriendsPivot: Migration {
    public static var entity: String {
        return "user_friends"
    }
}

I added the friends property to User:

var friends: Siblings<User, User, UserFriendsPivot> {
    return siblings()
}

Now, I'm seeing the following error on the line with return siblings():

Ambiguous use of 'siblings(related:through:)'

I tried to replace it with:

return siblings(related: User.self, through: UserFriendsPivot.self)

...without any luck.

I know that the two code snippets should work, because I straight-up copied them from another siblings relationship I built between Event and User that is working just fine.
The only difference I'm seeing is that I'm trying to build a relationship between the same models.

Is this a Vapor bug, or just me doing something wrong here?

Environment

• Vapor Framework version: 3.0.0
• Vapor Toolbox version: 3.1.10
• OS version: 10.14.3

[jbl8084]

Is MySQLPivot somehow a ModifiablePivot already?

[LinusGeffarth]

Don't think so. For attaching, I had to have it conform to ModifiablePivot. @jbl8084

[grundoon]

The issue here is that in a same-Model (User-User) siblings relation, Fluent cannot infer which sibling you are referring to – the sides need to be specified.

extension User {
    // the friends of the user
    var friends: Siblings<User, User, UserFriendsPivot> {
        return siblings(UserFriendsPivot.leftIDKey, UserFriendsPivot.rightIDKey)
    }

    // the users who have friended the user 
    var friendOf: Siblings<User, User, UserFriendsPivot> {
        return siblings(UserFriendsPivot.rightIDKey, UserFriendsPivot.leftIDKey)
    }
}

The other same-Model consequence is that you will not be able to use the attach convenience method to add to the pivot table, and need to manually create instead:

let pivot = try UserFriendsPivot(user, friend)
pivot.save(on: req)

(There are other approaches to work around this, I just find these straightforward ways above the easiest to use. Specifying the sides and reversing the key positions to obtain the inverse relation are the important concepts.)

[LinusGeffarth]

@grundoon, thank you for your answer, I was able to compile the project w/ the code you provided.
Can I use isAttached and detach, or do I have to work around those too?
If so, what's the equivalents?

[grundoon]

@LinusGeffarth Off the top of my head, I believe any method that accesses through the keyed Siblings<Base, Related, Through> relations you've defined above (such as isAttached, detach and detachAll) should work right out of the box.

user.friends.isAttached(friend, on: req)
     ^^^^^^^

[LinusGeffarth]

Yeah, I've found that both methods do work. Thanks for your help! @grundoon