def METHOD_NAME(apps, schema_editor):
    InventorySource = apps.get_model("main", "InventorySource")
    '''
    The Django app registry does not keep track of model inheritance. The
    source_vars_dict property comes from InventorySourceOptions via inheritance.
    This adds that property. Luckily, other properteries and functionality from
    InventorySourceOptions is not needed by the injector logic.
    '''
    setattr(InventorySource, 'source_vars_dict', VarsDictProperty('source_vars'))
    source_vars_backup = dict()
    for inv_source_obj in _get_inventory_sources(InventorySource):
        if inv_source_obj.source in FrozenInjectors:
            source_vars_backup[inv_source_obj.id] = dict(inv_source_obj.source_vars_dict)
            injector = FrozenInjectors[inv_source_obj.source]()
            new_inv_source_vars = injector.inventory_as_dict(inv_source_obj, None)
            inv_source_obj.source_vars = yaml.dump(new_inv_source_vars)
            inv_source_obj.save()