Retrieving the affected row count for a query is awkward

[0xTim]

To get a query's affected row count, it is currently necessary to retrieve the metadata from the database handle and unwrap the optional, adding considerable boilerplate logic, typically of this form:

extension SQLQueryBuilder where Connection == MySQLConnection {
    public func runReturningAffectedRows() -> Future<UInt64> {
        return connection.query(query) { _ in  }.map {
            guard let metadata = (self.connection as MySQLConnection).lastMetadata else {
                throw MySQLError(identifier: "metadata", reason: "Connection did not return metadata after successful query.")
            }
            return metadata.affectedRows
        }
    }
}

This also smacks of thread-safety issues; while it does not actually exhibit any, it feels iffy for the metadata of a given query to be associated with the database itself. There should be a more consistent and query-bound way to receive the row count. As well, it should be possible to retrieve said count from other databases than just MySQL - both SQLite and PostgreSQL are known to have such values available.

[tanner0101]

Agreed, this would be a nice addition. First step will be adding support for fetching affected row counts from SQLite and PSQL.

[tanner0101]

Track here vapor/fluent-kit#298