I am having issues with relationship (n to m)

[eluser]Unknown[/eluser]
I am trying save a relationship table and this sent to me the following error:

Fatal error: Class name must be a valid object or a string /website/fuel/application/libraries/Datamapper.php on line 2514

This is the function in the controller

Code:

public function add_field($enumeration_id, $fields)
    {
        // OK: get enumeration
        $enumeration = new Enumeration();
        $enumeration->where('id', $enumeration_id)->get();

        $field = new Field();

        // OK: validate type
        // if(is_json($fields))
        // {
        //     $fields_array = json_decode($fields, true);
        //     $fields->where_in('id', $fields_array)->get();
        // }
        // else
        // {
        // }
        $field->where('id', $fields)->get();
        // TODO : maybe!! validate data of fields

        // OK: save N to M
        if (!$enumeration->save_fields($field->all))
        {
            add_error($enumeration->error->all);
            return FALSE;
        }

        return TRUE;
    }

Models:

Code:

class Enumeration extends DataMapper
{
public $model = 'enumeration';
public $table = 'enumerations';

// Insert related models that enumeration can have more than one of.
public $has_many = array('field');
}

class Field extends DataMapper
{
    public $model = 'field';
    public $table = 'fields';

    // Insert related models that Field can have more than one of.
    public $has_many = array('enumeration');
}

Any idea about this?