getFieldForeignKeys example

    if (!empty($field_schema['indexes'])) {
      $schema['indexes'] = $this->getFieldIndexes($field_name, $field_schema, $column_mapping);
    }

    if (!empty($field_schema['unique keys'])) {
      $schema['unique keys'] = $this->getFieldUniqueKeys($field_name, $field_schema, $column_mapping);
    }

    if (!empty($field_schema['foreign keys'])) {
      $schema['foreign keys'] = $this->getFieldForeignKeys($field_name, $field_schema, $column_mapping);
    }

    // Process the 'id' and 'revision' entity keys for the base and revision     // tables.     if (($table_name === $base_table && $field_name === $this->entityType->getKey('id')) ||
      ($table_name === $revision_table && $field_name === $this->entityType->getKey('revision'))) {
      $this->processIdentifierSchema($schema, $field_name);
    }

    return $schema;
  }