Generate a resource for :meth:`_begin`.

def to_api_repr(self):
        """Generate a resource for :meth:`_begin`."""

        source_refs = [
            {
                "projectId": table.project,
                "datasetId": table.dataset_id,
                "tableId": table.table_id,
            }
            for table in self.sources
        ]

        configuration = self._configuration.to_api_repr()
        _helpers._set_sub_prop(configuration, ["copy", "sourceTables"], source_refs)
        _helpers._set_sub_prop(
            configuration,
            ["copy", "destinationTable"],
            {
                "projectId": self.destination.project,
                "datasetId": self.destination.dataset_id,
                "tableId": self.destination.table_id,
            },
        )

        return {
            "jobReference": self._properties["jobReference"],
            "configuration": configuration,
        }