`n+1` problem with eager loading models in different databases (connections)

[wojo1206]

What problem does this feature proposal attempt to solve?

I found n+1 problem with eager loading models in different databases (connections). Currently, the code will not eager load entitles that live on two different connections. I debug and tracked the problem to this specific line at src/Schema/Directives/RelationDirective.php:

if (
    $this->lighthouseConfig['batchload_relations']
    // Batch loading joins across both models, thus only works if they are on the same connection
    && $this->isSameConnection($relation) //  <-- Right here.
) {

Which possible solutions should be considered?

I don't know what is the rationale for this limitation but that should be OK by default to eager load models on different connections. To solve this problem we can either eliminate this limitation or introduce new config variable that allows to override this check at $this->isSameConnection($relation), something like:

    /*
    |--------------------------------------------------------------------------
    | Batchload Relations
    |--------------------------------------------------------------------------
    |
    | If set to true, relations marked with directives like @hasMany or @belongsTo
    | will be optimized by combining the queries through the BatchLoader.
    |
    */

    'batchload_relations' => true,
    'batchload_relations_only_on_same_connections' => true,

[spawnia]

The reason for this is right there in the comment:

Batch loading joins across both models

By joins i mean an actual SQL JOIN statement. A single database statement involves the tables of both models.

[wojo1206]

Hmm, joins across multiple databases are fine as long as the databases are on the same server instance. The concept of multi-database system in Laravel means that you have set multiple connections even if your databases are on the same instance.