How to manage BelongsToMany relationship in Laravel ?

[AxelReviron]

I'm working with API Platform for Laravel and encountering difficulties managing BelongsToMany relationships. While BelongsTo and HasMany relationships work seamlessly by sending IRIs in the request body, BelongsToMany doesn't seem to follow the same pattern.

I'm trying to add users (participants) to a trip when creating (POST) or updating (PATCH) a trip.

Here's the Trip Model:

#[ApiResource(
    operations: [
        new Get(),
        new GetCollection(),
        new Post(
            rules: StoreTripRequest::class
        ),
        new Patch(
            rules: UpdateTripRequest::class
        ),
        new Delete()
    ]
)]
class Trip extends Model
{

    use HasFactory, HasUuids;

    /**
     * The attributes that are mass assignable.
     *
     * @var list<string>
     */
    protected $fillable = [
        'label',
        'description',
        'distance',
        'duration',
        'is_public',
        'geojson',
    ];

    protected $casts = [
        'is_public' => 'boolean',
    ];

    public function author(): BelongsTo
    {
        return $this->belongsTo(User::class);
    }

    public function stops(): HasMany
    {
        return $this->hasMany(Stop::class);
    }

    public function participants(): BelongsToMany
    {
        return $this->belongsToMany(User::class, 'user_trip', 'trip_id', 'user_id');
    }

    public function luggages(): HasMany
    {
        return $this->hasMany(Luggage::class);
    }

For BelongsTo (author) and HasMany (stops) relationships, sending IRIs in the request body works as expected during a POST or PATCH operation:

{
    "label": "My Trip",
    "description": "",
    "distance": null,
    "duration": null,
    "is_public": true,
    "geojson": null,
    "author": "/api/users/01977e42-9412-713c-861c-93cfbdd9c077",
    "stops": [
        "/api/stops/01977e42-98cd-73fb-9fae-e76b52fed90f",
        "/api/stops/01977e42-98c1-73c7-b35e-d3790fba6ba0"
    ],
}

However, when I try to do something similar for the participants (BelongsToMany) relation by including a list of user IRIs:

"participants": [
        "/api/users/01977e42-9412-713c-861c-93cfbdd9c077"
    ]

I receive the following error:

"type": "/errors/500",
    "title": "An error occurred",
    "status": 500,
    "detail": "SQLSTATE[42S22]: Column not found: 1054 Unknown column 'participants' in 'field list' (Connection: mysql, SQL: update `trips` set `participants` = [{\"id\":\"01977e42-9412-713c-861c-93cfbdd9c077\",\"pseudo\":\"lexa\",\"email\":\"[email protected]\",\"email_verified_at\":\"2025-06-17T14:19:41.000000Z\",\"created_at\":\"2025-06-17T14:19:41.000000Z\",\"updated_at\":\"2025-06-17T14:19:41.000000Z\"}], `trips`.`updated_at` = 2025-06-17 15:54:00 where `id` = 01977e42-9560-700a-8ccd-32ad2957f207)"

It's not correctly interacting with the BelongsToMany relation to manage entries in the user_trip pivot table.
Thank you in advance for your help!

[AxelReviron]

Based on my understanding of the documentation, subresources in API Platform are designed for GET operations so that is not what I need here.
I attempted to use a DTO with a StateProcessor to send new participants and their permissions (view/edit) for a Trip. However, I encountered a 403 error because the ResourceAccessChecker evaluates the Gate with the DTO instance instead of the actual Trip model, causing the authorization to fail.
This behavior made me realize what @soyuka explained here.
I also looked into the default 'PersistProcessor' for Laravel and found that BelongsToMany relationships are not handled.
At this point, it feels like I’m fighting against the framework’s design. I’m considering creating a dedicated controller with a custom route to handle this.
Am I missing something, or is this the correct approach in API Platform with Laravel?