use Illuminate\Database\Eloquent\Builder;
// Retrieve posts with at least one comment containing words like foo%...
$posts = App\Post::whereHas('comments', function (Builder $query) { $query->where('content', 'like', 'foo%');
})->get();
// Retrieve posts with at least ten comments containing words like foo%...
$posts = App\Post::whereHas('comments', function (Builder $query) { $query->where('content', 'like', 'foo%');
}, '>=', 10)->get();

Event::with(["owner", "participants" => function($q) use($someId){ $q->where('participants.IdUser', '=', 1); //$q->where('some other field', $someId);
}])

use Illuminate\Database\Eloquent\Builder;
// Retrieve posts with at least one comment containing words like code%...
$posts = Post::whereHas('comments', function (Builder $query) { $query->where('content', 'like', 'code%');
})->get();
// Retrieve posts with at least ten comments containing words like code%...
$posts = Post::whereHas('comments', function (Builder $query) { $query->where('content', 'like', 'code%');
}, '>=', 10)->get();

enough database theory
look at the most often example – when User has a Profile
=> they are related with one-to-one relationship one user has only one profile
class User extends Model
{ function profile() { return $this->hasOne('App\UserProfile'); }
}

$movies = Movie::whereHas('director', function($q) { $q->where('name', 'great');
})->get();

$roles = App\User::find(1)->roles()->orderBy('name')->get();