code: public static function statement(Database $database, array $definition, $modelName, $tableName, $foreignName, $foreignTableName)
      {
          $tableDefinition = $definition[$modelName];
          $foreignDefinition = $tableDefinition['links'][$foreignName];
          $foreignTableDefinition = $definition[$foreignDefinition['model']];
          $from = array();
          $to = array();
          foreach ($foreignDefinition['fields'] as $field => $linked) {
              if (isset($tableDefinition['fields'][$field]['name']) === true) {
                  $from[] = $tableDefinition['fields'][$field]['name'];
              } else {
                  $from[] = Helper::codifyName($modelName) . '_' . Helper::codifyName($field);
              }
              if (isset($foreignTableDefinition['fields'][$linked]['name']) === true) {
                  $to[] = $foreignTableDefinition['fields'][$linked]['name'];
              } else {
                  $to[] = Helper::codifyName($foreignDefinition['model']) . '_' . Helper::codifyName($linked);
              }
          }
          $sql = 'ALTER TABLE ' . $tableName . ' ADD CONSTRAINT fk_' . $foreignName . ' FOREIGN KEY (' . implode(', ', $from) . ') REFERENCES ' . $foreignTableName . '(' . implode(', ', $to) . ')';
          if (isset($foreignDefinition['update']) === true) {
              $sql .= ' ON UPDATE ' . strtoupper($foreignDefinition['update']);
          }
          if (isset($foreignDefinition['delete']) === true) {
              $sql .= ' ON DELETE ' . strtoupper($foreignDefinition['delete']);
          }
          $statement = $database->getMasterLink()->prepare($sql);
  
          return $statement;
      }
docs: Prepare and return according PDO statement.
  
  @param  Database     $database         database
  @param  array        $definition       schema definition
  @param  string       $modelName        model name
  @param  string       $tableName        table real name
  @param  string       $foreignName      foreign key name
  @param  string       $foreignTableName foreign table real name
  @return \PDOStatement $statement prepared query