Many-to-Many relations in Vapor 3 with Fluent

Question

I was wondering how to create many-to-many relations in Vapor 3 using Fluent and FluentMySQL as described in the Vapor 2 docs

Sadly, the docs for Vapor 3 have not been updated yet and the implementation for the Pivot protocol has changed.

Here is what I'm trying to do: I have two classes, User and Community. A Community has members and Users can be members of multiple Communitys.

Currently, my code looks like this:

import Vapor
import FluentMySQL

final class Community: MySQLModel {
    var id: Int?
    //Community Attributes here
}

final class User: MySQLModel {
    var id: Int?
    //User Attributes here
}

extension Community {
    var members: Siblings<Community, User, Pivot<Community, User>> {
        return siblings()
    }
}

However, this causes the following compiler errors:

Cannot specialize non-generic type 'Pivot' and Using 'Pivot' as a concrete type conforming to protocol 'Pivot' is not supported.

I have seen that there is a protocol extension called ModifiablePivot but I don't know how to use it since there is no documentation or sample code anywhere.

Any help is appreciated. Thanks in advance!

Answer 1

Fluent 3 no longer provides a default pivot like Pivot in Fluent 2. What you should do instead is create a type that conforms to Pivot. There are some helper types in FluentMySQL for this.

final class CommunityUser: MySQLPivot {
    // implement the rest of the protocol requirements
    var communityID: Community.ID
    var userID: User.ID
}

Then use the CommunityUser in place of Pivot<Community, User>.

Answer 2

I had the same issue and both of the answers weren't sufficient for me, so i'm just posting my solution which worked for me.

// in CommunityUser.swift
import Vapor
import FluentMySQL


final class CommunityUser: MySQLPivot {

    typealias Left = User
    typealias Right = Community

    static var leftIDKey: LeftIDKey = \.userID
    static var rightIDKey: RightIDKey = \.communityID

    var id: Int?
    var userID: Int
    var communityID: Int

    init(id: Int?, userID: Int, communityID: Int) {
        self.id = id
        self.userID = userID
        self.communityID = communityID
    }

}

// CommunityUser didn't conform to MySQLMigration
// This simple line solves the problem
extension CommunityUser: MySQLMigration { }

For me CommunityUser also needed to be migrated in the Database.

// in configure.swift
migrations.add(model: CommunityUser.self, database: .mysql)