code: public function getParentRelationship($tableName)
      {
          return $this->fromCache($this->cachePrefix.'_parent_'.$tableName, function () use ($tableName) {
              return $this->getParentRelationshipWithoutCache($tableName);
          });
      }
docs: If this table is pointing to a parent table (if its primary key is a foreign key pointing on another table),
  this function will return the pointed table.
  This function will return null if there is no parent table.
  
  @param string $tableName
  
  @return ForeignKeyConstraint|null