Use relatedKey instead of getKey in order to manage not-standard belongsTomany

[alberto-bottarini]

I've got this scenario.

Two models:

• Rule
• Persona

A Rule has a belongsToMany relationship with Persona:

public function personas()
{
  return $this->belongsToMany(Persona::class, 'feature_rule_personas', 'rule_id', 'persona_code', 'id', 'code');
}

This is the pivot table structure:

id, rule_id, persona_code

I would like to have in the pivot table not the Persona ->getKey but a custom key, in this case the code column: Laravel will allow me to do this using some extra parameter in belongsToMany.

Without this PR, plugin will use sync method using id column and not the right code column, inserting wrong data in database.

I added a $resource->{$parentModel->$relationship()->getRelatedKeyName()} in order to get the right relatedKey.