Product::getCategoryId PHP Method

    public function getCategoryId()
    {
        return is_object($this->category) ? $this->category->id : null;
    }

 /**
  * Filter the query by a related Product object
  *
  * @param   Product|PropelObjectCollection $product  the related object to use as filter
  * @param     string $comparison Operator to use for the column comparison, defaults to Criteria::EQUAL
  *
  * @return                 CategoryQuery The current query, for fluid interface
  * @throws PropelException - if the provided filter is invalid.
  */
 public function filterByProduct($product, $comparison = null)
 {
     if ($product instanceof Product) {
         return $this->addUsingAlias(CategoryPeer::ENTITY_ID, $product->getCategoryId(), $comparison);
     } elseif ($product instanceof PropelObjectCollection) {
         return $this->useProductQuery()->filterByPrimaryKeys($product->getPrimaryKeys())->endUse();
     } else {
         throw new PropelException('filterByProduct() only accepts arguments of type Product or PropelCollection');
     }
 }