'Allocates a range of key IDs for this model class.
Args:
size: Number of IDs to allocate. Either size or max can be specified,
not both.
max: Maximum ID to allocate. Either size or max can be specified,
not both.
parent: Parent key for which the IDs will be allocated.
**ctx_options: Context options.
Returns:
A tuple with (start, end) for the allocated range, inclusive.'

@classmethod
def _allocate_ids(cls, size=None, max=None, parent=None, **ctx_options):

    return cls._allocate_ids_async(size=size, max=max, parent=parent, **ctx_options).get_result()
