code: public function orderBy( $columns, $direction = 'asc' ) { // @codingStandardsIgnoreLine
  		if ( is_string( $columns ) ) {
  			$columns = $this->argument_to_array( $columns );
  		}
  
  		foreach ( $columns as $key => $column ) {
  			if ( is_numeric( $key ) ) {
  				$this->statements['orders'][ $column ] = $direction;
  			} else {
  				$this->statements['orders'][ $key ] = $column;
  			}
  		}
  
  		return $this;
  	}
docs: Add an order by statement to the current query
  
  ->orderBy('created_at')
  ->orderBy('modified_at', 'desc')
  
  // multiple order statements
  ->orderBy(['firstname', 'lastname'], 'desc')
  
  // muliple order statements with diffrent directions
  ->orderBy(['firstname' => 'asc', 'lastname' => 'desc'])
  
  @param array|string $columns   Columns.
  @param string       $direction Direction.
  
  @return self The current query builder.