/**
  * Parses and processes a model join into a database join expression
  * @param zibo\library\orm\query\ModelJoin $join
  * @return zibo\library\database\manipulation\expression\JoinExpression
  */
 private function parseJoin(ModelJoin $join)
 {
     $condition = $join->getCondition();
     $condition = $this->parseCondition($condition);
     $table = $join->getTable();
     $table = $this->addTable($table->getModelName(), $table->getAlias());
     return new JoinExpression($join->getType(), $table, $condition);
 }