$this->db->select('field1.id AS id1, field1.weight AS weight1, field2.id AS id2, field2.weight AS weight2')
								->from('res_form_fields AS uj_field1')
								->join('res_form_fields AS uj_field2', 'field2.form_id', 'field1.form_id')
								->where('field1.id', $field_id)
								->where('field2.weight > uj_field1.weight')
								->limit(1)
								->get();

+++++++ OUTPUT +++++++

SELECT `uj_field1`.`id` AS `id1`, `uj_field1`.`weight` AS `weight1`, `uj_field2`.`id` AS `id2`, `uj_field2`.`weight` AS `weight2`
FROM `uj_res_form_fields` AS `uj_field1` JOIN (`uj_res_form_fields` AS `uj_field2`) ON (`uj_field2`.`form_id` = `uj_field1`.`form_id`)
WHERE `uj_field1`.`id` = 2
AND uj_field2.weight > uj_field1.weight
LIMIT 0, 1