Simple Example

service.php

<?php

use Doctrine\Common\Persistence\ObjectManager;

class Lead
{
  protected $POST_Data;
  
  protected $_em;
  
  public function __construct(ObjectManager $em) {
    $this->_em = $em;
  }

  public function hasLegalFields()
  {
    $legalFields = $this->getLeadFields();
  
    //Compare $POST_Data against the $legalFields array
    foreach ($POST_Data as $key => $value) {
      if ($key if in $legalFields array) {
        //do nothing
      } else {
        unset($POST_Data[$key];
      }
    }
  }
  
  public function getLeadFields()
  {
    $repository = $this->_em->getDoctrine()
      ->getRepository('AppBundle:LeadFields');

    $leadFields = $repository->findAll();
    
    //process these objects into an array containing ONLY the leadField attribute and return it.
  }

}

services.yml
services:
    lead_bundle.lead:
        class: LeadBundle\Lead
        arguments: [@doctrine.orm.entity_manager]
        tags:
            - { name: kernel.event_subscriber }
            
//Class can be accessed like this in your controller or wherever your container is injected at
$this->get('lead_bundle.lead')->hasLegalFields()