Siblings relationship between same models in Vapor

Question

I have a User model which I want to add a friends property to. Friends, are supposed to be other Users.

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.

What can I do?

Answer 1

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

    // users who are friends with this user
    var friendOf: Siblings<User, User, UserFriendsPivot> {
        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.)

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^{as answered by grundoon}