Attach existing record to eloquent hasMany / belongsTo relationship

I have a model SalesArea which hasMany Regions. Regions belongTo one SalesArea.

class SalesArea extends Model
{
    public function regions() {
        return $this->hasMany('App\Region');
    }
}

class Region extends Model
{
    public function sales_area() {
        return $this->belongsTo('App\SalesArea');
    }
}

Due to some changes in organization, the parent (SalesArea) is being created after the Regions have been created. I am trying to associate the Regions with their associated Sales areas when the Sales Area record is created.

I created a migration to add sales_area_id to the regions table, and they are currently all null.

I have a select box that has the various IDs of Regions and I would like to associate the regions with a Sales Area at once with an array of ids. None of the association methods are working.

$area = SalesArea::create($request->all());

$area->regions()->attach([1,2,3]); // BadMethodCallException with message 'Call to undefined method Illuminate\Database\Query\Builder::attach()

$area->regions()->add([1,2,3]); // BadMethodCallException with message 'Call to undefined method Illuminate\Database\Query\Builder::add()'

$area->regions()->sync([1,2,3]); // BadMethodCallException with message 'Call to undefined method Illuminate\Database\Query\Builder::sync()'

$area->regions()->associate([1,2,3]); // BadMethodCallException with message 'Call to undefined method Illuminate\Database\Query\Builder::associate()'

I am stumped on how this is supposed to work. Regions already exist, I just need to add their relationships to their associated sales area.

via Chebli Mohamed