ModelDataset evaluates queryset during import

[Benjamin Wohlwend]

Specifically, DatasetMetaclass evaluates the queryset twice:

if not opts.model and not opts.queryset:
    raise NoObjectsException("You must set a model or non-empty "
                             "queryset for each Dataset subclass")
if opts.queryset:
    queryset = opts.queryset
    model = queryset.model
    new_class.queryset = queryset
    new_class.model = model

This can lead to hard to debug problems, e.g. when the database tables don't exist yet. Instead, these if-conditions could specifically check if the queryset is not None:

if not opts.model and opts.queryset is None:
    raise NoObjectsException("You must set a model or non-empty "
                             "queryset for each Dataset subclass")
if opts.queryset is not None:
    queryset = opts.queryset
    model = queryset.model
    new_class.queryset = queryset
    new_class.model = model