Query Relationships Across Multiple Database Connections

Query related models located on different database connections using whereHas() without cross-database join errors.

When your application partitions data across multiple database connections (such as separating analytics logs, multi-tenant tenants, or payment microservices onto different database servers), standard SQL joins between connection tables fail.

Eloquent's whereHas() directly handles relations across different database connections.

Defining Models on Separate Connections

namespace AppModels;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\HasMany;

class Tenant extends Model
{
    protected $connection = 'main_db';

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

class ActivityLog extends Model
{
    // Stored on a separate logging database server
    protected $connection = 'logging_db';
}

Querying Across Connections with whereHas()

use App\Models\Tenant;

// Eloquent handles the cross-connection query correctly
$activeTenants = Tenant::whereHas('activityLogs', function ($query) {
    $query->where('created_at', '>=', now()->subDays(7));
})->get();

Summary

• Enables relational querying even when models reside on distinct database servers.
• Uses WHERE EXISTS queries scoped by database name.
• Eliminates manual cross-database ID querying loops.