def get_virtual_sources(self, model):
        sources = []
        for source in self.sources:
            target_field = self.resolve_source(model, source)
            if target_field is None:
                sources.append(source)
        return sources
Returns a list of strings that are joined together to go after the
dataset/ETHPy150Open pivotal-energy-solutions/django-datatable-view/datatableview/columns.py/Column.get_virtual_sources--crossover with--dataset/ETHPy150Open cloudera/hue/desktop/core/ext-py/Django-1.6.10/django/db/models/sql/compiler.py/SQLCompiler.get_from_clause