'Convert xml model_class element to a galaxy model class
(or None if model class is not found).
This element is required and only the first element is used.
The model_class string must be in ALLOWED_MODEL_CLASSES.'

def parse_model_class(self, xml_tree):

    if ((xml_tree is None) or (not xml_tree.text)):
        raise ParsingException('data_source    entry    requires    a    model_class')
    if (xml_tree.text not in self.ALLOWED_MODEL_CLASSES):
        raise ParsingException(('Invalid    data_source    model_class:    %s' % xml_tree.text))
    model_class = getattr(galaxy.model, xml_tree.text, None)
    return model_class
